I keep sucking at the elite four so I'm going to start a new game.

What starter should I get, and what pokemon should I have in my party and where can I get those Pokemon?

well I would choose Charizard
but I think you should choose a starter you can rely on
mewtwo,zapdos,moltres,and articuno are good choices to

Mewtwo= you must beat the game and go to curealean cave
zapdos=powerplant
moltres=one island cave
articuno= seacave full of bolders and ice on an island
suicune/raikou/entai=
you must have bulbasuar as a starter to get entai
you must have charmander to get suicune
and you must have squirtle to get raikou
you must have beaten the elite four and just walk aroun the place and you'll end up getting suicune

the way I beat the elite four was with trust and friendship + luck in my pokemon and I train them and never give up.

i like charmander. get a jolteon/raichu, pidgeoet/dodrio/fearow, slowbro/king and maybe a sandslash. If you like the 3 birds get them Also get a nidoqueen/nidoking