I agree that PJ's are a great way to try out a pattern, etc. for the first time. If they work out perfectly, you get to luxuriate in them. If you make a mistake, it's okay because you're the only person who sees them.
In fact, that's how my Night and Day Pants pattern started--I designed and made a pair of knit PJ's for myself, which I enjoyed so much I decided to figure out how to vary it so that I could use the same pattern for pants I could wear out of the house. [And then share it, of course! I suppose I should post the link, but that seems a bit like shameless self-promotion:
http://www.johnnymangoseed.com/collections/women/products/night-and-day-pants . They have gotten good reviews from beginning sewers--if you do try them, let me know what you think!]
As far as which fabric to use, using what you have is great because it's inexpensive! If you have a choice amongst a variety of fabrics, I would go with a middle weight such as mid weight cotton interlock. Very lightweight knits can be tricky because of the way they curl at the cut edges. Though I adore them, I'd also stay clear of rayon knits, etc. for your first knit project.
Have fun! I delayed and delayed on sewing with knits, but I am so glad I took the plunge!
Enjoy!
Pamela