Before setting up BuddyPress
The most important step to building a community is defining your "Community Purpose". While this seems like an obvious statement, more often than not, people who start an online community fail to pick their niche or fail to stick to it.
You shouldn't just start an online community about anything and everything, mainly because the market is already dominated with these types of sites; unless you are completely mad or backed by billions of dollars—taking on Facebook or Twitter wouldn't be recommended.
Picking your market and determining what the purpose of your community will be, will make the process of creating your community significantly easier, finding community members easier and will help search engines know exactly what your website is about.
Simply stating that you want to have the biggest network with X users is not a valid Community Purpose. Instead you want to be as detailed as you possibly can within 140 characters; something like "Building a community...