If you're not ready to plunk down $250 or more for a bicycle, you may be able to get something decent used, but if you're also that novice with little or not knowledge about bikes, you're going to need that experienced cyclist friend or family member. He or she can help ensure the bike is the right size and of decent quality and condition.
People selling lower- and middle-range bikes are often unclear about what they have and what size it is. It's very common to see an ad for a "26-inch men's mountain bike" for instance. 99.99% of the time that means a bike with 26-inch wheels, not a 26-inch frame. Frame size is what determines whether the bike will fit or not. (My wife is 5 feet tall and I'm 5'8"; we both ride mountain bikes with 26-inch wheels. Hers is a 13-inch frame and mine is a 16-inch. A 26-inch frame is for professional basketball players.)
The seller may be able to give you some idea of the bike's fit potential; ask how tall the rider is. Your experienced friend can help if you get a photo via the web or e-mail. That can help you screen out the bikes that definitely won't fit.
When you think you have a bike you'd like to see in person, you'll need your friend again. He or she can look it over for mechanical issues you might not catch. The frame and the wheels are the parts you'll want to inspect the closest; other parts are less critical and cheaper to replace. Look for cracks, rust (on steel frames), and ripples in the paint. Those ripples mean the frame was seriously stressed in a crash; walk away from that one. Make sure wheels are reasonably true and the bearings aren't crunchy.
Make sure the bike fits by straddling the top tube and lifting the handlebars straight up. For a road bike with a top tube parallel to the ground you should be able to lift the front wheel one to two inches off the ground. If it's a mountain bike, a hybrid or comfort bike, or a road bike with a sloping top tube, you should be able to lift the front wheel two to four inches. Have your friend set the saddle to the correct height so you can go for a ride together; he or she should be able to see any obvious fit or mechanical problems that way.