Hello and welcome c: !
I'd recommend you go to training mode and figure out how his aerials, tilts, grabs, smash attacks and specials work because everything in Pikachu's arsenal is useful.
His best approach options are fair, dair, RAR uair, grab, quick attack and maybe nair, so Pikachu has plenty of options to approach. Never ever use dash attack as an approach option because if you miss you'll be punished hard.
Pikachu's grab range isn't that great so you have to get used to it as well, I know Pit/Dark Pit have an INCREDIBLE grab range, while Pikachu's is kinda limited. Each throw has it's purpose and you need to decide which throw to use depending on % and stage positioning.
Something that's troublesome for newer Pikachu mains is getting the kill so you gotta learn how to kill with him, his main kill moves are his smash attacks, especially up smash and side smash and thunder. Pikachu has some setups for that for example fffair into up smash and up throw to thunder.
Pikachu also has an INSANE offstage game and ledge guard game which can help you get some early kills so try being confident offstage as Pikachu, his recovery is too good so you can always come back from the depths of hell.
Also it's very important that you master his quick attack angles so you can recover and use it offensively.