That's an excellent question Turtle(: I would see if you can open up about little things to her/him first. By this I mean sharing something that if, Heaven for bid if he/she told someone, you honestly wouldn't care. Then I personally would tell them a little bit more an ask him/her about there personal experiences with whatever topic you are talking about, so that you build a healthy, honest open relationship. :)
You need to be extremely careful with any adult with whom you build a relationship outside of your family. You should never be alone with them for any extended period of time, and make sure your patents know them.
Here's the concern .... Young adults go through years of having adult feelings coupled with young sensibilities. It makes for a pretty confusing time and sometimes young people get hurt by getting involved with something that is WAY over their head. If you need a mentor seek a professional. There are very strict guidelines to protect the venerable. Of course there are no guarantees but you should be safer.
If its a relationship as a friend or acquaintance I'd say once the level of interaction between you 2 reaches a point where you're comfortable around each other then opening up will come naturally for both of you. Opening up from the beginning can show a level if trust but it can also make some people uncomfortable.