If you are familiar with a particular 3d modeling software you shouldn't have any probs (but I think you still need Oxygen to import models to work in OFP).
If you have no experience whatsoever (like me not that long ago), then you can go straight for Oxygen.
You can get it and other usefull things here:
http://www.flashpoint1985.com/breatheThere are some very usefull tutorials to get you started here:
http://ofp.info/brsseb/http://www.dc3d.co.nz/Tutorials/index.htmAs for finding out how to do everything, you will just have to try out a few things and do some experimenting.
If you get stuck there is always the forums here and over on the official site here:
http://www.flashpoint1985.com/cgi-bin/ikonboard301/ikonboard.cgi?s=3c35bc596380ffffThey are an amazing source of information and insight.
Always try the search feature first before posting, chances are someone has already asked what you need to know.
Have fun! ;D