Getting started
First of all, you might want to read the FAQ.
That covers some of the basics you'll need to get going.
That being done, your next step is to get your thumb and fingers
(at least the index and middle ones) operating as independent entities.
Your thumb will carry the bass; your fingers the melody.
To aid in that endeavor, I've put together some examples of common
alternating bass patterns for you to practice. They start with a
rather simple one (#1), then progressivly add melody notes and
get a little more involved.
Once you feel comfortable with these pattern and can play them with some common open-string chords, try one of the easier arrangements. I'd recommend Sweet Bye And Bye or This Land Is Your Land as good places to start.