A good place to start out is always as a citizen. Get to know the basics, find out what you like, then eventually progress through the factions as you go.
A good pointer would be CWU or UM. They're both primarily roleplay orientated factions that exist with the single purpose of creating more roleplay environments for others to interact with.
The Civil Workers Union are something like your janitor, construction workers, industrial production line and maintenance all in one.
Union Medical are the medics operating in the city, taking care of every citizen, loyalist or even covert rebel alike, with plenty of avenues to discover if you're into that stuff instead.
As the next step, Civil Protection of course. Though it comes with some decent responsibility, so you should only apply for it if you feel like you've got the basics down, a good understanding of how the server works and if you've founded your knowledge regarding the faction and how it works. Best way for that is to just observe how some whitelist holders play, maybe talk to them.
Once you got that, then you can think of applying for OTA, because on nebulous, it won't be as easy as say, your average other HL2 server to get the whitelist, as it's a little more sophisticated than that.
