If it feels more comfortable to push with your front foot, that's fine too. This is called pushing Mongo. It can get in the way of tricks in short board skateboarding, but for longboarding, if it works for you, use it. Most people don't.
The longer you longboard, the better you will get at getting the most out of your pushes, so don't worry too much right now if it seems you have to push a lot!
Push around a little more, and feel where your center of gravity is. When rolling, notice that the lower you set your center of gravity, the more comfortable it is to ride. This is important.
Longboard Basics - Find a SlopeNext, you'll want to find a gentle slope to ride down - and I mean gentle! You'll be riding down hills in just a minuet - first, find a slight slope. This can be a parking lot with a slope to it, a road without traffic, maybe a footpath across campus...
Now, get on your longboard, and try riding it down the slope. The first time, don't even push. Just get on and let gravity pull you down.
Next, try pushing once, and riding down. Ride down the slope until you feel comfortable.