Step 4: Fit
They generally don't charge for this, but again, if you go for a bike fitting, and don't buy a bike from that shop, you should really buy your accessories and/or service from them. Bike shops operate with a pretty small margin.
As a super quick and simple rule of thumb, when you stand over a bike frame (in front of the saddle) with your shoes on and feet flat on the ground, there should be at least 1 inch, preferably 3 or 4, between the top bar and the beginning of your... special parts. For one thing, if you were to be in a minor crash and fall forward off the saddle, you don't want to be impacting a big steel (or aluminum, or carbon) bar of bike frame. This is also an indication that the frame more or less fits you. If, when you straddle the bike, the top tube is actually touching you, that bike is much too big for you, and no matter how much you love it and no matter how good the price is, it just isn't the bike for you. I'm sorry.
On the opposite end, you can use the seatpost to judge if a bike is too big.
When you ride, your knees should be 99% of the way straight at the bottom of each pedal stroke (not 100%, or locked-out, but almost). If you don't extend your legs all the way, (imagine how it looks when an adult rides a tricycle meant for kids), you will end up hurting your knees.
The seat post (the part that attaches the saddle to the frame) should have a line on it marking the maximum its meant to be extended (it may or may not say words to that effect). By loosing a screw or bolt at the place where the frame clamps the post, you can raise or lower the seat height. If the post is at its highest (at the line) and when you ride the bike your legs are not extending fully, then the bike is too small for you. You can always buy a slightly longer seatpost if its close, but if the seat needs to be higher than about 10 inches above the frame (assuming the top tube of the bike frame is horizontal, more on that soon), chances are the rest of the dimensions are too small for you anyway.
Of course, many bikes don't have a straight horizontal top tube running from the handlebars to the seat. The sloping top-tube or the frame that used to indicate "girl's bike" (in order that the bike could be ridden in a skirt or dress) has become common on mountain bikes and commute oriented bikes and many hybrids for both genders, as well as continuing to be common for female specific road bikes. With a sloping top tube you can't just stand over it and measure the distance between your body and the frame to determine fit, and the seat-post will have to be extra long. In that case you just draw an imaginary line (or better yet, use a level and some string or a broomstick or something) where the top tube would be if it went straight across.
There is really a lot more to it than that, (such as the length of the top tube and the angle of the seat tube) but to keep things simple, 1-5 inches between you and the bike frame (or where the frame would be if it were straight) with your feet flat on the ground is about the best approximation there is.
Once you find one bike that fits, you can check the frame size (usually printed on the seat tube) and have a rough idea of what other bikes will also fit you - generally mountain bikes and hybrids will be measured in inches, road bikes in centimeters. But be aware that the numbers can vary between bike styles, brands, ages and depending on whether the top tube is horizontal or not.
So now you have some idea of what kind of bike you are looking for, and what size it needs to be.