don't get hit
pichu has a decent ground game which you need to maximize in most matchups. he is really good at juggling and gimping.
learn how to move as fast as possible with pichu and time nair for whatever you need. chaingrabs are also important to know how to do. use dash dancing, wavedashes, and shield dropping to the best of your ability.
pichu's matchups against relevant characters are all really bad. the only ones that are anywhere close to salvageable are marth and captain falcon but they beat pichu solidly as well. the only good matchups pichu has are against other low tiers.
i'll assume you mean neutral. outplay the other person. bait people into doing stuff and dash dance to pressure people with pichu's threat zone around him, the area where he can nair, up tilt, down tilt, and grab. make sure to have people respect that area. don't let yourself get overwhelmed, because that leads to you getting hit.
pichu's fresh nair is +1 on shield if done as late as possible, and staled nair is +0. there is also a bit of disjoint on his nair around his butt, allowing you to beat out other moves. up tilt and up air also have a lot of disjoint. instant wall jumps (aka doraki jumps) are useful for playing around the ledge. pichu can also land on the ground in the middle of agility with no lag if the end of the move is near the ground. you can do this from the ground by jumping and using agility immediately after you are in the air.
stages are matchup dependant. smaller stages benefit you more if you need to keep characters locked down, while larger stages are better if you want to use your mobility more. stages with closer blast zones are better if you need early kills, and the opposite is true if pichu can gimp well. i think the worst stage for pichu in general is FD because it takes away his platform game, making his predictable neutral more restricted.