It differs from person to person but there are a few basics. You want your cadence in the 80-100 range if possible (the number of revolutions your crank does in 1 minute). Newbies tend to ride in a gear that's too high and quickly wear themselves out and drain their energy. You shouldn't be spinning at a cadence of 120 the whole route but also not look like you're doing a Sunday drive through the neighbourhood. Shift gears as you would with your car. If you slow down or stop, gear down before hand or while slowing down. When you pull off the bike is in a light enough gear to move forward as quick as possible. Always look at the trail a few meters infront of you. If you're on a decline and approaching an incline, especially if it's sudden, shift down before hand. Try and avoid shifting on the front chainrings will doing an incline and even if you shift on your rear cassette while on an incline, try and keep as little as possible tension on the chain when shifting. If your chain is under a lot of tension on an incline and you shift, you can hear the pain your chain is going through as well as the teeth on your chainrings and cassette, so when shifting on an incline, pedal a bit slower (remove some tension). Then your components will last longer.
