What do I do?

EdWorf

Newbie and just got my first character maxed out. How do I make him better?

Lurker

choose a new champ to max out. rinse and repeat for all eternity.

Shrimkins

If he isn't awoken yet you could try to do that. Either by pulling the same champ again in a crystal or by using an awakening gem.

Once he's awoken you can increase his signature level by pulling him over and over in crystals or using signature stones.

But all that doesn't really make champs that much stronger. Some champs get much better when awoken and others don't have any noticeable effect.

The best thing is to grow your roster. Level up as many champs as you can. Most champs serve some sort of purpose in the game so having a big bag of tools is very helpful.