Sheeva has a few options.
If you're midscreen, you can for, low-grab, walk (dont dash), 2, 1, WalknGrab, easy combo and does around 35%+ damage.
For something more damaging you can go for lowgrab, B2, 1+2, cancel into fireball. This is quite easy to do and does high damage. If you punch someone out the air, you can follow up with, B2, 1+2 as they land.
One of her strongest combos is low grab (and this can be done, midscreen), B2, 1+2, Walk and Grab. Sometimes the grab whiffs, but if you land it, you'll get around 49% damage I think (it's been awhile since I used her so not sure if that is correct), but its VERY HIGH DAMAGE for no bar use. I do know the EX WalknGrab will usually always connect, of course, this means using one bar.
You can also use her anti air grab in a variation of the above combos and that usually has high priority if you have someone in a juggle state.
Catching people with her low grab is hard enough as it is, but the B2, 1+2 cancel into special, is a good "punish" combo. Her 1, 2, B1 is also a combo launcher that sets up some other nice stuff.
Sheeva is one of those characters that's fairly easy to use, has high damage, but hard to win matches with.
I really wish they could buff her low grab somehow. Make it either faster or make it that you can cancel combos into a low grab.