Well I'm a relative novice compared to most of the people here. Maybe I can help. If I get anything wrong, somneone's sure to straighten me out
Do you plan to just run one train, or more than one at once?
Do you have a sense of what scale you want?
The least expensive choice for an engine and cars would probably be a Bachmann starter set. Bachmann mostly offers "western" narrow guage stock in 1:22 scale. Here's an example:
www.ridgeroadstation.com/istar.asp
The Bachmann track, though, is no good for outdoors.
The next least expensive would probably be an Aristocraft starter set. Aristocraft stuff is mostly less "western" looking and mostly standard guage models in 1:29 scale. Aristicraft has a huge product line and an excellent web presence. The company responds quickly to questions. Their track is excellent
USA Trains makes a few starter sets but I never see them anywhere
LGB starter sets are very high quality and expensive
You don't have to get a starter set, you could buy individual pieces
Track is expensive! I've used LGB and aristocraft track. LGB track is very sturdy; Aristocraft track feels a little less sturdy but has a better system for joining pieces. I prefer the Aristocraft track. USA trains makes track just like it.
You'll need to figure out how to lay the track. The simplest way is proably to dig a shallow trench the size of the track, then fill it with small gravel and "float" the track on top. It has advantages and disadvantages. I like the "ladder" method--if you search the archives of the track and roadbed forum for "ladder" you'll find a lot of info.
You'll need to power it--a starter set will come with an adequate transformer, but only just adequate. There are a bunch of ways of powering the trains and they get complicated quickly
Here are two good places for general information
www.btcomm.com/trains/primer/index.htm
www.girr.org/girr/tips/tips.html
www.girr.org/girr/tips/tips.html