Well, it's kind of a broad question, but I'll do what I can to help. First, read up on the forums. Check out pictish's slightly dated but still useful guide, and my matchups list, to learn about what pika's capabilities and limitations are. Browse through the other topics to see what discussion has gone on, as much on here can be helpful.
Then, check out some videos. Get DC++ and connect to the smashboards hub (there's a guide to doing it in the melee discussion boards I think) and see what pika vids you can find. A few have also been posted here in the pika boards by some smashers, and there are a few in the video boards if you're dedicated enough to search for them. Most are hosted on youtube or googlevideo, so you can also search those sites (generally searching for player name + "pikachu" + "v." will get you results). Watch what the pika player does, what his playstyle is, how he reacts to the opponent, and any intersting things you see.
Go into practice mode or start a 1v1 vs. a lvl 1 comp and practice some stuff. Make sure you can do all the basics fairly consistently (short hopping, l-cancelling, all that jazz). Then play around with anything you think you need practice with or want to try out. If you see a particularly cool or common combo on the videos that you want to try, see if you can pull it off. Or play around with some of his moves to make sure you have a full grasp on what it does: its windup time, hitbox, lag time, knockback & damage properties, etc. The goal is to get comfortable with pikachu as a character so that in tournament play you're paying more attention to the opponent than you are to yourself to make sure you're not messing up.
Once you're confident in your tech skill, play some people. See what of pika's works and what doesn't on human opponents. If you keep getting owned after the same move, figure out what you're doing wrong or if there's a better way to approach the situation. Study your opponent while you play them and see what habits they have and be thinking of how to exploit them. Mix up your game too, always keep them guessing as to what's going to come next, and then punish them when they guess wrong.
I know this is a very general response to a general question, but there is a lot of information on these boards if you look. If you have any more specific questions (i.e. "how do you usually go about approaching a Fox?" or "is there any good way to practice varying my approach angle with quick attack?") feel free to ask in this thread. I'll do what I can to help.
