I've had a threesome with my girlfriend before (FFF), it's not all that hard of a concept to despite how daunting I imagine people would think it is. As stated before, communication is key, but you should also consider how to go about it. Try not to be overly focused on a single member, it's easy to make someone feel left out. Switch up roles, don't be passive about taking the lead with your partner or the other girl taking the lead with him, so he can focus on the third member, that way someone always gets something out of it. Also take it slow, you're most likely gonna be with a girl you've never been with, so make sure to make that external party feel comfortable and get them in the mood first and foremost. They're gonna be the most nervous, probably anyway. Other than that just be careful with who you chose.
Beyond that it's just regular sex with an additional member, once you get into the flow of things after foreplay, kissing and getting everyone in the mood to go all the way, you'll get caught up in the flow and treat it like you would normally with your partner. Finally, as suggested look up some amateur porn threesomes, should give you a good idea of what to do if you don't know how to start or how to work together.
I probably don't need to add this but remember protection, worst thing that could happen is you get a random girl with some STD or STI she doesn't know about and that ruins the good time, dental dams and condoms are a must.