Advice Needed Which platform?

I would like to set up an community online. Which software is the best to use? And then with theme I need?

If you want to make a community you can use IPB. (easy modification, Skin is found everywhere.

I honestly recommend Xenoforo. It is made by people who orginally created Vbulletin back in the days when it was actually good (3.0-4.0).. IPB is also a good choice. If you want free alternatives, phpbb and mybb are probably the top.