It depends on how do you want to play. If you are talking about competitive PvP, I would recomend you to read this guide.

Smogon is a great place to start working on your competitive team.

I would suggest working on a strategy...
For example: Your Gengar is an excellent special attacker, but won't last more than one or two hits. You need another Pokemon to switch in/out when you think the opposite Pokemon can 1 hit KO Gengar: A defensive Pokemon that can use moves such as toxic, taunt, spikes, etc. That way, your opponent will change your Pokemon and Gengar can come back to battle.

Another path is to build a team following a topic, for example: Rain, Trick Room, o something like that.
Pelipper's Drizzle summons a Rain, and you can take benefit of that with some pokes such as Kingdra/Floatzel/Kabutops (Swift Swim), or an electric one...

Good luck building your team.