First of all, doubles or singles? If it's doubles, I can show you my old rain team and let you try playing with that. It's not 100% but it's built alright. Can't help you much with singles.
General advice: Get used to playing with and against different pokemon. Get an idea, at least, of their strengths and weaknesses. Decide on what sort of team you want to build. There's a lot of different teams out there that utilizes different strategies. One pokemon carelessly added in can wreck with your team's synergy.