I think that when you are ready to be in a relationship you will know. For me - it's the feeling of not wanting to be apart from the person, they become your best friend and lover. You miss them when you are away from them and think about them without realising it. For instance when you go shopping you see things that they would like rather than looking for things for yourself etc.
Mind you I get a little wrapped up in love. Everyone is different.

Personally the 'first time' should be with someone special that you are in a relationship with and care about deeply. Sex can be an emotional experience - especially the first time.