Canada's involvement in the Revolutionary War comes up often enough on this sub that it is addressed by several sources in the FAQ, one of which is a previous answer of mine. I am copy-pasting my previous response here with some minor changes for clarification, and with some further information about Newfoundland and Florida, which I did not address previously:

Colonies in Newfoundland and Nova Scotia were founded by the English in the 1600s primarily as seasonal fishing colonies. Fishermen from England and Scotland would come over for several months each year to fish and then return to the British Isles afterward. The climate in Newfoundland and Nova Scotia was not entirely hospitable for agriculture, so settlers who came to stay in America permanently tended to go to one of the "Thirteen Colonies" to the south.

But Nova Scotia, and (what are now) the other Maritime provinces of Prince Edward Island and New Brunswick were in dispute between England and France. France had settlements on these islands as well, administered by them as the colony of "Acadia". So it wasn't only climate that discouraged settlement, it was the precarious political situation there, too, that made the Thirteen Colonies to the south a more attractive destination to English/British colonists.

The rest of present day Canada was firmly in control of the French and founded as New France in the 1600s. The British didn't take possession of what are now Nova Scotia, Prince Edward Island, New Brunswick, Quebec, Ontario, and all lands west until after the outbreak of the French and Indian War in 1754, and of course, that possession wasn't recognized by France until the Treaty of Paris of 1763 that brought the French and Indian War to its conclusion.

At the end of the war, the Acadian population was largely deported, mostly to either Quebec, Louisiana, or one of the French colonies in the Caribbean. To aid in maintaining British control of the land and keep the French (and their allied First Nations) out, the British government offered free land in the Maritimes to settlers from the Thirteen Colonies willing to relocate. About 8000 American transplants responded to the call, and these transplants made up the majority of the English speaking population of Canada before and during the Revolutionary War.

So at the outset of the Revolution, "Canada" as we know it today was primarily made up of the small English town of Halifax (founded in 1749) and English-speaking farmers in the surrounding countryside, as well as the French towns of Quebec and Montreal and the farmers in the surrounding countryside there along the St. Lawrence River.

It was at this time that the seeds of unrest began to blossom in the Thirteen Colonies against British Parliament. The French-speaking Canadiens also distrusted the British government who they now found themselves ruled by. They were particularly angered because the Brits were essentially excluding Catholics from serving in local government offices by requiring an oath of office that Catholics would be unwilling to make. And being French, the vast majority of the population was Catholic.

In an effort to make sure that the political unrest in the Thirteen Colonies didn't spread to Canada, the British Parliament passed the Quebec Act in 1774, which not only allowed the Canadiens to freely practice Catholicism, even among office-holders, but it also expanded the colony's borders to include the land south of the Great Lakes and west of the Appalachians ("Ohio Country", now the states of Michigan, Ohio, Kentucky, Indiana, Illinois, and Wisconsin). This law actually had several purposes: it kept the French Catholics from joining the rebellion, while it also demonstrated to the Thirteen Colonies that if they did not cooperate with Parliament, they would not be treated well, and if they did cooperate, the would be treated well.

The Quebec Act placated Quebec but roused the Thirteen Colonies, Virginia in particular since they had claimed "Ohio Country" since before the French and Indian War began (and a dispute over its possession had largely started that war). Thus, the Quebec Act was cited among the "Intolerable Acts" that were seen by the Continental Congress as justification for rebelling, and was mentioned in the Declaration of Independence in the twentieth grievance ("For abolishing the free System of English Laws in a neighbouring Province, establishing therein an Arbitrary government, and enlarging its Boundaries so as to render it at once an example and fit instrument for introducing the same absolute rule into these Colonies").

The Continental Congress did send letters to the local governments of all the Canadian colonies inviting them to join the Congress and, thus, the Revolution, but they did not receive a favorable response. However, Quebec mustered together two regiments of Patriots who fought alongside Washington's Continental Army.

The Continental Army tried to invade Quebec in 1775, led by Benedict Arnold before he switched sides, but this invasion was unsuccessful. Likewise, Massachusetts militia men attacked Nova Scotia at the Battle of Fort Cumberland in 1776, and were joined by some pro-Patriot Nova Scotians, but were also unsuccessful. As one might imagine, the population of Nova Scotia was largely Loyalist. After all, these were mostly people who had answered the British government's call for settlers little more than a decade earlier, many coming after the political turmoil had already begun. They weren't going to risk giving up the land they'd just been granted by the British government, and had relocated to Nova Scotia to claim, only to lose it in an unsuccessful revolution. At the end of the Battle of Fort Cumberland, many of the most fervent local Patriots were forced out of Nova Scotia, and the colony remained firmly loyal to Britain for the remainder of the war.

Officially, though, Nova Scotia remained "neutral", but the British Navy launched many of their expeditions from Halifax, so it maintained a British military presence for much of the war.

Prince Edward Island and Newfoundland were even smaller than Nova Scotia, but the same issues that affected Nova Scotia affected those two colonies as well. That is, most of their population had been seasonal fishermen before the 1750s, and the permanent English language population didn't really start to grow until the tail end of the French and Indian War. One estimate has the population of Newfoundland being less than 8000 permanent residents by the end of the 1750s, and still only about 12,000 permanent residents by the end of the 1770s, which would mean at least a third of the population was newly settled, having taken up cheap or free land grants from the British government. Many of these were Americans, but there were also a lot of Irish, too. This would mean their population was only about half of Georgia's and less than a tenth of South Carolina's, which was only the eighth most populous of the Thirteen Colonies. Newfoundland was tiny in comparison.

In 1775, British Parliament passed the Restraining Acts which forbade the citizens of the New England colonies from fishing along the Nova Scotian and Newfoundland coasts, which they had previously been able to do. These acts were extended to the other Thirteen Colonies if they joined Massachusetts' rebellion, and so it was an effort to pit the Thirteen Colonies against each other, and it briefly worked in some of the colonies, but ultimately, the Continental Congress decided to stand together and passed a bill that embargoed exports from the Thirteen Colonies to Newfoundland and Nova Scotia in retaliation against the Restraining Acts. This was particularly devastating for Newfoundland, as the colony had little agriculture and their primary trading partners were the Thirteen Colonies. They were now completely reliant on trade with Europe (mostly Great Britain and Ireland), so whatever sympathies that the small population of Newfoundland had--which weren't that great to begin with--were completely diminished thereafter. British loyalty was secured once the British sent a military presence to Newfoundland to secure the trading relationship.

And then on St. John's (Prince Edward Island), the British had only taken full control of the island in 1758, and expulsion of the French occurred thereafter, so the population of the colony at the outset of the American Revolution was miniscule. An estimate from the time in 1770 said that there were only 150 English families living on the island. It was small enough throughout the war that isolated American pirates could raid the island to plunder supplies from the locals and even take hostages back to the Thirteen Colonies with them, beginning with the Raid on Charlottetown in 1775. After further raids, naval officer Timothy Hierlihy was sent to Prince Edward Island in 1778 to fend off further raids. Despite losing three of the six ships in a storm while crossing from Nova Scotia to Prince Edward Island, the Hierlihy's presence was successful. The Americans did not raid the small Prince Edward Island population again.

(...cont'd...)