Try the character specific boards for Pikachu
http://smashboards.com/forumdisplay.php?f=70
Also here's a matchup description for Pikachu from the sticky, unfortunately Sheik is one of Pika's abolute worst matchups.
"Sheik: She'll chain throw you. And then ftilt/dtilt you. And then fair you. And then you're on your next stock. Use the stage to escape being chainthrown if you can (Di to uneven surfaces on some stages, DI and walltech off walls on others, DI to platforms on FoD), or DI off the edge (and try to avoid the fair). If you get a chance to do something, dsmash then follow with aerials, hit-and-run with nairs, and bthrow->uair spike if possible. Pika actually has some decent aerial comboing ablility on sheik, he just has difficulty getting in to start the combo and not get punished for it. Watch out for her needle approaches. If you get caught in your sheild, you'll get grabbed, and that's very bad. Draw her into the air if you can, and then stay below her as she doesn't have that much to do against you from that position except ff'd aerials, most of which are outprioritized by uair if you're below her. She's a bit annoying to edge-guard, but it's definately doable. And if you do get a couple aerials on her while she's off the stage, it's usually enough to kill her. Just watch out for her fairing while she's recovering to prevent this. When she's ledgehogging you, try to upB to a platform if at all possible. She'll usually ledgehop bairs when you return, so going for the stage usually isn't that safe. It's not incredibly unbalanced once you get past the chaingrab. But, ftilt->fair is just ********."