First of all, keep playing the people that are way better than you, as well as the people worse than you. Secondly, you yourself need to determine what to focus on. Focus on utilizing all of your options and reading your opponents.
Mixups are great, but 50/50s aren’t nearly as useful as "This guy is mashing on wakeup, I will meaty him every time and get nice combo and then block the one desperation DP/ultra and kill him for it.
Online with randoms is awful, people abuse lag and think they’re good. Play people you meet in regional matchmaking (a subforum of SRK for finding people) or at least play people online that you meet on SRK. Some randoms will be good, some will be retarded.
Everyone can do OS tech, but even characters with really good cr.lk’s can get blown up for it.
If I personally was going to pick one character right now for the metagame, I would play Fei Long, Zangief, . Look at top 8’s from tournaments yourself though.
That’s too many characters for a new player, drop one.
Adon can pretty much fight anyone, as can Akuma. They have a few bad matchups, but nothing unsurmountable or as frustrating as Zangief vs Chun Li. Dhalsim loses pretty strongly 4-6 to a lot of characters though, unless you really enjoy playing him I don’t see the appeal.
If your character can get unblockable’d by Cammy, you need to consider how much you really like that character.
This tier list is serviceable, although I disagree with some numbers. Seth has an average matchup advantage of 5.42, Vega has an average of 4.87. Note that even the D tier characters aren’t significantly worse than the A tier. Every bad matchup matters.