If you are serious about
online poker but also new to the game, then 100$ is a reasonable amount of money to deposit. The reason is, it gives you a reasonable shot at not having to deposit again, if you quickly become a winning player and follow good bankroll management. 100$ will give you 50 times the full buyin for 2NL cash games, 67 times the buyin for 1,5$ SnGs and 100 buyins for 1$ MTTs. Those are reasonable numbers to work with, and these are the limits, you should start with, until you have gained experience and build a track record.
As for which format, thats totally up to you. The best advice is really to try them all and then pick the one, you enjoy most. Because if you dont enjoy playing, its not going to be a success. That also goes for variations like 6-max vs. full ring, Zoom if you choose cash games, and maybe even try some alternative games like PLO. At some point its best to specialise, but in the beginning you need to try different games out to find your path. Ideally not from session to session, but maybe give each game format a week or month, and then try something new.