Lots of resources on Youtube. Number 1 rule is control the space, spam fireballs if you have to, some characters have way better reach than others. Force them to make that mistake of jumping over and punish them. Footsies are a good way to keep your opponent cornered (crouch with a light kick). Anti-airs will prevent you from getting combo'd from above. If they try to grab you and you grab back, the damage is far less than if you didn't try to grab at all, it may even cancel the grab.
Hope that helps.
