How do you last longer as a virgin?

Hi , I'm 17 and I really need some advice about this . I met a girl , well we knew each other but we didn't speak to each other for 4 years . Her boyfriend is out of country for some months and we started seeing each other quite often and I have a hunch that she really likes me for not going out with her friends to hang out at my place , in the park etc. She told me that she is cheating on him , she just got out of another relationship , but she will always keep him because she loves him ... eh not gonna comment on that ... For some days we started to get closer and closer and I am not making a move because I'm afraid that I'm gonna suck for the first time , and she has allot of experience . I don't want to really fall in love with her , I am really up to this ''open relationship'' for how many months does it take , I can do this without really falling for her and I need this type of relationship. But I can't take it slow , she is kinda str8 forward and for example I guess that if we hook up she will want sex , and I don't want to make a bad impression the first time. So any tips on how should I last the first time? real tips , not trollish ones :P