Ok so lets break some of this down!

was the United States military building up their ground troops, planes, ships etc before we officially declared war on Japan and Germany?

Oh Absolutely! Though much of it was limited by what the public and Congress were willing to fund or allow. The draft and compulsory military service for instance actually began in 1940 as a means to build up the Army somewhat and add a larger base of men with military training, up to 100k men at a time could be called up for service for a fixed length of time.

While the Navy was seeing steady expansion on all front thanks to what are known as the Vinson Acts, the Bill passed in July of 1940(The Two Ocean Navy Act) was set to fund a 70% expansion fo the Navy to show the scale of the buildup that was planned. The act set up the construction of almost 20 air craft carriers of various sizes(including the first of the famous Essex class ships) a half dozen battleships and scores of smaller vessels and auxiliaries and aircraft.

And if so how much development was going into a war we were not yet involved in?

While the plans sound and were grand at the time, we must admit only real modest progress had been made towards getting the Army and Navy ready for a war footing come Pearl Harbor. The Army was still small and not yet fully modernized to the needed levels for a modern war, while relatively few of the new ships were actually out to the fleet, and new models of airplane were only just becoming operational or even entering production.

And finally why did we wait so long to help our European allies rather than enter the war after the Japanese attacked us?

Im not sure what you mean here? Germany declared war on the US within a week after Pearl Harbor on their own with the US then responding in kind. While the policy of Europe first was never seriously questioned even by those fighting in the Pacific, there was just an immediate need to staunch the bleeding in that theater through really the first year of the war, while buildups for offensives against Germany took months to prepare and ready.

However even before Pearl Harbor, beyond Lend Lease I think the argument is pretty easy that the US was actively engaged in the Allied cause to a degree. Taking over the occupation of Iceland for instance freed up British troops who had been there, while the Neutrality Patrol was escorting Allied convoys from Canada to the middle of the Atlantic and was in a low key shooting war with U-Boats weeks before Pearl Harbor.

And of course the fact that outside of the USSR and China our Allies in the European Theater and Pacific were the same.

Is it true the United States military had guerilla units dropped into Germany pre United States involvement?

Lord no if the US had such effective and ready clandestine units even before Pearl Harbor the war could have been quite different. But no Hell it took a year to even realize the British Commando model was effective and form the first Ranger unit.

In addition to /u/DBHT14 's answer, the US was effectively in a state of war with Germany after Roosevelt's order on September 11, 1941 to attack U-Boats on sight, following an incident involving USS Greer a few days earlier. The first USN ship hit by a U-Boat was USS Kearny on 17 October, and Reuben James was sunk on 31 October.

Even prior to the shoot on sight order, US vessels would track German ships and U-Boats and Roosevelt extended the 'neutrality zone' far beyond US shores.