What I usually do is pick one pokemon (usually a mega) and build your team around covering its weaknesses. For example, start with Flygon (Mega). It is weak to Ice, Flying, Dragon, and Rock. So add Infernape to take care of Ice and Rock, and Delta Gardevoir to take care of Flying and Dragon. Then, build the rest of the team however you want, just be aware of common weaknesses and remember that picking pokemon you like is more important than making the most competitive team.
Here is an example:
Flygon
Infernape
D. Gardevoir
D. Roserade
Feraligatr
Aegislash