You can get free maps at
http://www.gpsfiledepot.com. They've got Topo, trails, public land, and many other great maps.
You can use Garmin's "Mapsource" or "Basecamp" software to load the maps you need for a specific trip. I use a Mac so I have to use the Basecamp software, but it's pretty good and they're both free. You can find those apps on Garmin's web site.
As others have said, reading the manual is the place to start. When I got my first GPS I played with it a lot before I used it for backpacking, and the first few trips I used it on were on familiar ground. I tested the "Track" and Track Back" features and setting waypoints to see how accurate they were so I knew what to expect, and played with all the features.
I found that I could easily lose a signal in steep canyons and thick forests. Your eTrex is better than my old eMap, but that can still happen to you, so you need to find those limits and be prepared to work around them.
I also found that at its best it could be very accurate in showing me my track and pointing which way to get where I wanted to go, or not at all accurate, so it took a bit of practice to really make good use of the info it provides.
My first GPS came with a "Quick Start Guide", and I carried that with me for a long time (more than a year) while learning how to use it. Your's probably has one too, but you may have to print it out yourself. I strongly recommend you do that. The menus on the eTrex can be confusing, at least at first, and you need to learn how to get to the info you'll use most. The Quick Start Guide will save you hours of frustration trying to find what you're looking for.
As others have also said already, bring a printed map and compass too, and use the GPS to locate yourself on the printed map, and use the printed map to give you the bigger picture. With the tiny screen on those handhelds you can lose your point of reference pretty fast when you zoom in or out very far.
Never put yourself in a position where you must depend on the GPS to get where you need to go, they can and do fail. I've bought three Garmin GPS units over the past 10 years or so and two of them died with no warning or reason.
All that said, that is a pretty capable GPS with some great features and the more you know about using them the more you can learn about navigating without one. They are a great learning tool if you use them for that purpose. Used correctly you can hone you map skills and locate some great spots that might otherwise be hard to find, and find the shortest route when you need or want to. But there's also a lot of other info they provide, like Sun and Moon rise and set times, solunar tables for fishing, hunting, or just viewing wildlife, locating water sources like springs and creeks, finding distance traveled and average miles per hour. Plus, it's a lot of fun to upload your GPS trip data into your Mapsource or Basecamp software when you get back from a trip and review your track.
I think the eTrex models are a great starting point for learning to use a GPS. I liked my old eMap better because it had a bigger screen and a simpler menu, and I really like the "Oregon 200" model I have now, but the eTrex is a good choice to start off with. The software isn't as easy to use as I'd prefer, but it has a lot of features so you can learn a lot from it.