My answer below presupposes that the question refers to Anglophone sailors

how hard was it to become a sailor? That would depend on a number of factors, probably the foremost of which is whether a state of peace or war existed at the time. During wartime the Royal Navy significantly increased its strength so there were plenty of employment opportunities for the inexperienced sailor in both the Navy and merchant ships. In peacetime the situation was very different: thousands of men who had gained seafaring experience during the last war found themselves unemployed and competing for a limited number of places. In the immediate aftermath of a war there tended to be a trade surge as merchants hurried to move stockpiled commodities, and to some extent that mitigated the employment problem, but it wasn't a complete solution and it tended not to last more than a year or so. One prospect for a young man who wanted to go to sea would be to get himself apprenticed to a sailor. An apprenticeship usually lasted seven years and offered experience and sustenance, but rarely any pay. Officers and tradesmen (coopers, carpenters etc) frequently had apprentices, but so too sometimes did foremastmen ('regular' seamen).

How about in a position of power? Define 'power'. The company of almost every kind of ship was essentially hierarchical in nature, so power wasn't hard to obtain. On a small merchant ship, say, the master snarled at the mate, the mate swore at the boatswain, the boatswain beat the seamen, the seamen beat the boys, and the boys kicked the dog. Achieving some kind of 'rank' would depend on the skill and experience of the seaman, and the service in which they worked. In the navy there was room for promotion, so someone keen and good and who managed to catch the eye of their superiors could reasonably expect some advancement, slow but regular. In, say, the coasting trades, crews were small and the opportunities to rise through the ranks were correspondingly limited.

Was it a good life with good opportunities? A lot of the popular image of early-modern sailors focuses on the bad sides of life - the poor food, the floggings, the back-breaking work. All of those things existed, but they need to be seen in context. The quality of sailors' food was poor by modern standards, and frequently lacked variety due to the requirement that it had to be preserved on long voyages, but the quantity of food supplied to seamen was generally greater than someone of a similar social class could expect on land, and usually with a much higher proportion of meat. Floggings and beatings happened, but were not necessarily more common at sea than on land, and most sailors accepted that a certain amount of violence was concomitant with the good order upon which they knew their lives depended. Pay for a seaman was at least equitable to pay for a labourer on land, but during wartime merchant ship masters offered significantly higher wages in an effort to attract seamen, so although there was the additional danger of death or capture by the enemy, the rewards were good. Frequently seamen were allowed to conduct a little trade on their own account, purchasing small amounts of foreign commodities that could be sold for a profit elsewhere on top of their pay. On naval vessels such opportunities were scarcer, and wages were consistent in peace or war, but the larger crews relative to the size of ship meant that the work was easier.

Would just joining to work on the ship lend itself to one day become the ships captain? Almost never. There was a definite divide between those 'before the mast' and the quarterdeck officers. Someone who started their seagoing career as a deck hand might reasonably have hopes of rising to the rank of boatswain or quartermaster, but would be unlikely to progress to mate or captain. Being a captain required navigational expertise which was beyond the scope of most seamen. There are a few cases of seamen rising to become captains, but incredibly rarely. The vast majority of captains began as junior deck officers or apprentices to officers.