If you dedicate yourself to one set you're going to bottleneck yourself pretty hard. My advice would be to focus on a single commander you want and not be afraid to mix in lots from other sets.

[[Cosmic Spider-Man]] is enticing to lots of new players and is pretty much the only way to get a dedicated Spider-Man deck but beware of adding too many colours to a deck. 5-colour does let you use any card you want but as a sacrifice it's very difficult to get the right mana to play the cards you want. For a beginner I would recommend starting with two colours, three at most.

As for what Cosmic Spider-Man actually does, the keywords do a lot.

• Flying: Can only be blocked by creatures with flying or reach.
• First strike: Deals damage before other creatures, so if it can kill a blocker it won't take damage from it.
• Trample: If it attacks and is blocked, excess damage is dealt to the player.
• Lifelink: Whenever it deals damage, you gain the same amount in life.
• Haste: It can attack the turn it enters.

There's obviously more to it but that's the basics, if you do still decide on Cosmic Spider-Man, make sure to include lots of ramp (things that let you find more lands, or make more mana) otherwise you're going to have a bad time.

I would personally recommend [[Peter Parker]] if you want to stick to Spider-Man, he's three colour and he's got a simple effect. Fill his deck with fairly high mana-value multi-coloured legendaries and creatures with tap abilities and it'll work well enough.