Hi /vp/

After beating the elite 4 I have decided to try and be a bit more competitive with my post game team. I want to do some online battling and I know I need to EV/IV train but I figured picking out my 6 Pokemon should be my first concern. Is there a site that help with my team choosing? Or can the 6 really be any 6? (Obviously not all one type)

If it helps I am thinking of using at least a Lucario so far.

Thanks for any replies