Bit like cars really, what are you intending to use it for, how much do you want to spend,
Just a few tips, get an air tank to refill it, get one that's regulated, go to a local gunshop and have a handle of some,
choose between a wooden stock or a tactical one,
.177 is probably the most common, and easier to re-sell if you want to upgrade.
Also join and try your local gun club, they will have members who will guide you, and they may have some rifles for sale.
There are thousands for sale out there, so do your research, and try as many as you can,
When you decide which one you want, go on tinternet and ask for the problems relating to the one you want,
All this before you actually buy one.