I would say it’s best to pick at least one “God” character (storm, magneto, sentinel, or cable).
Even if you don’t like any of those characters, you should have at least one of them in your team, because if you want to be playing competitevly, you’ll eventually need to switch to some top tier teams. Otherwise, you will have such an uphill battle that you’ll hit a wall eventually.
When you pick your characters, try to focus on teamwork. It’s generally good to have one decent AAA (anti air assist, such as captain commando, ken, Cammy, cyclops). It’s also good to have a pressure assist (sents Ground type, Doom’s Anti Air type, etc). Learn to use those assists as many newer players ignore assists and get trampled because of it.
If you’re interested, here’s a breakdown of top tier teams and their assists:
Mag\Storm\Psy (projectile, expansion, AAA): I would say this is a very technical team, uses Psylocke for her antiair assist mostly in combos with magneto, and to start infinite and snapback combos.
Mag\Storm\Sent (projectile, expansion, projectile): very similar to the above, sent on Projectile is mostly for assisting during magneto combos. However, sent is much stronger on point than psylocke, but his assist doesn’t set up infinites\snapback combos like hers does.
Storm\Sent\Capcom (projectile, ground, anti-air): This build has a lot of potential but isn’t nearly as technical as the above teams. Capcom’s AAA is considered by many to be the best in the game - it has great startup, some invincablity, and reaches to the very top of the screen. It can also be very combo friendly, for both storm and sent. This also frees up Sent to use his ground assist, which is great for pressure and gives storm cover to start her rushdown game.
Cable\Sent\CapCom (AAA, Ground, AAA). This is a great pressure team, Cable on point can control tons of space, and all it takes is one hole in his opponents offence and he can AHVB x4 (air Hyper Viper Beam four times in a row). Using Sent and CapCom as assists he can keep them at bay even more, and if he has to tag out his assist is pretty amazing too.
A couple other characters to consider:
Ken - his AAA assist is completely invincible and starts up immediately. This thing will go through just about any super, and is only vulnerable once he lands. He’s terrible on point though.
Tron Bonne - her Projectile assist has limited range, but IIRC it does the most damage of any assist - about twice as damaging as a rocket punch.
Stryder when paired with Doom can do a ton of chip damage with Stryder’s Oberos super and Doom’s AAA assist, usually you’ll see Sentinal as the third in that combo. You don’t see that as much as you used to, but it was very popular for awhile.
Also every once in a great while you’ll see someone playing Spiral and Sentinal. Together they have an infinite lockdown combo, meaning that they can keep someone in blockstun permanently until they die from chip damage.
Anyways, hope that helps for a primer. I’m sure others could go into greater detail than I, but that should at least get you started.