I've worked with probably well over a thousand novices in the last 15 years and there are definitely some important tips that will greatly help beginner riders.
Most important: If you have not ridden a bike in a few years it is not a good idea to start again with mountain biking unless you happen to be the athletic type. Just start riding a regular or hybrid bike, stay on roads. Build up your confidence, balance, and bike fitness first. Bike fitness is somewhat different then other types of fitness. Don't let the long times of sitting get to you. Try standing while riding in order to get a chance to stretch. Standing is an important novice skill that will be useful and make mountain biking safer for you
Standing while riding: Imagine yourself standing over the middle bar of your bike with both feet on the ground. Keep in mind that the bottom of your feet while on the ground are parallel to the ground and are relatively level to each other. Now while riding stand on the pedals making sure that feet are parallel to ground but that the pedals are on the same level plane and not one pedal down and one up. The key is that your trying to create a stable platform so your weight is equally balanced over the bike. If you try standing with one foot down and one up most of your weight will be on the down foot, this is not a safe way of standing. We will revisit this later
Brakes: In Mt biking brakes are important but are over rated. Their main function is to control your speed and rarely used to have to stop suddenly. So ride in a small area and get used to your brakes and how much pressure is required to slow you down but not bring you to an abrupt stop. In Mt biking your brakes make it possible to slow down enough to make turns but you do not want to completely stop. You can use your brakes often but just lightly.
Gears: Get used to you gears, some bikes shifters are very sensitive and it doesn't take much of a push or click to make a shift. Experiment and try to shift JUST one gear at a time and do not skip any gears, unless you are going down hill then it is possible to skip some. Efficient use of gears does take a while to learn. Lots of patience and practice is the key
There are often three front gears in Mt bikes, sometimes two and even one. So as a novice be sure that your front smallest diameter (size) gear is the one that you use mostly. The real challenge is to become really familiar with and use most or all those gears in the rear of the bike. As the trail terrain changes from flat to rolling or hilly terrain maintaining your forward momentum (speed) is completely dependent on how quickly you can anticipate and shift gears to adjust for the changes of elevation while climbing or descending. So the key is to shift your gears much sooner then you think and much more often then you ever have in the past.
You legs may begin to give a subtle hint that they are straining and usually by then you may have lost much of your forward momentum, try to shift just before that straining feeling sets in. The other reason to get used to that right hand shifter (right side) is so you will not get confused. its important that if you are climbing that you shift those rear gears in the correct direction ONE gear at a time. While climbing your chain is also climbing from smaller to bigger diameter gears. Steeper and faster the climbs the faster you need to shift gears one at a time.
Somtimes you may need to slow down to make a turn, especially if you have picked up speed on a downhill section, try to remember to shift your gear to a larger diameter chain ring in the rear and do this just before you resume pedaling again and you will be able to keep moving at a slower speed. As the bike slows down in order to maintain balance you need to keep those feet moving (spinning) this happens only when you shift to a gear that will keep you feet spinning and the bike will then move slow enough to maintain balance through tighter places. Gearing to maintain proper momentum while the terrain is changing is a challenge and we will revisit this again
Shifting Weight, Stand! And Deliver
to be continued