I would also recommend PQ canyon as a place for your first ride, just stay on the south side of the creek, as there are some tricky parts on the north side trail.

I also would recommend North of the Border Bikes the have a few rental bikes, and both Mike and Doug are very knowledgable guys who have been mountain biking for decades. They are also having a Demo Day this Saturday where Marin BIkes will have a large demo fleet of bikes that you can try out for free.