One thing that can be helpful, when talking to women or anyone else you feel tense around, is to come at the conversation from some sort of shared experience or interest. If you're meeting someone through a class, event, shared hobby, etc. that can be a topic to talk about in order to get a conversation started.
Starting a conversation doesn't need to be any more complicated than asking how someone's doing or asking them to talk about something they're interested in. That doesn't mean it won't feel awkward or scary, sometimes, but keeping things simple may cut down on the stress that you feel.
These advice columns might have some helpful thoughts as well: I Just Want a Relationship
and How Do I Let Go Of Feeling Sexually Unattractive?