Mountain Biking - New from Philadelphia surburb

Hi,

My boyfriend and I decided to start bicycling. Since we haven't been on a bike in say..15-20 years we just wanted an inexpensive starter bike. We both got one from Dicks Sporting Goods. That said, I want to be extra careful with the bike and maintain it the best I can.
I got an East Coast Realm 26" mountain bike.
I'm a little confused about the shifter, it's an Enzo Twist Shifter with 1-6 on the right shifter for the rear, and an H and L on the left for the front with about 10 dots between them.
Choosing one of them in the middle puts the front in 2nd.

Usually we see 1-2-3 so we're not sure how to properly shift this bike.

There are 3 gears in the front though.
http://www.finaltransmission.com/006a.jpg

Sorry for sounding and being such a newb.
The manual did not say much.

the 'L' means Low, that's the small ring on the crankset (this is what your pedals are attached to). If you shift it closer to the 'H', or High moves it into a bigger ring. This is a higher gear so if you're going slow it will be harder to pedal but allow you to keep up to speed if you want go fast.
It's the same, although reverse, on the back (this is called a cassette). The bigger the ring on the cassette the easier it is to pedal and the smaller the ring, the harder/faster.
Just don't over think it. Just go ride and you'll figure it out pretty quick. Have fun and ride the wheels off of them.

I don't know if this will go over your head, but the reason the left gear shifter has so many options is so that you can fine tune the position of the front derailleur to keep if from rubbing against the chain. For example: If your front gear is all the way to the left & the rear is all the way to the right, the piece of metal that moves the chain over would normally be rubbing against the chain in the front. Having a lot of small changes when you shift rather than three large ones allows you to move the position of the front derailleur slightly enough to keep it from rubbing the chain without shifting to another gear.

+1 to what Sam said. I would just add that you can hear if the chain is rubbing so it's obvious when you need to move it just a bit more. It's pretty intuitive, and you'll get a feel for it really quickly.

If you're doing legitimate mountain biking & not just cruising around the neighborhood, you'll also find that you rarely need to shift out of the 2nd gear in the front. The rear derailleur is the work horse.