Assuming you are talking about a mountain bike that is designed for off-road use, the most important factor in choosing a gear ratio is the terrain you'll be riding on. If you're mostly riding on flat ground with the occasional hill, you'll want a lower gear ratio so you can pedal more efficiently. A good rule of thumb is to choose a gear ratio that is equivalent to the number of teeth on your front chainring divided by the number of teeth on your rear cog. For example, if you have a 36 tooth front ring and an 18 tooth rear cog, your gear ratio would be 2:1. This would be considered a low gear because it's easy to pedal but not very fast. If you're mostly riding uphill, you'll want a higher gear ratio so you can maintain pedaling speed. In this case, you might choose something like 3:1 or 4:1. Ultimately, it's up to you to experiment and find what feels best for the type of riding you do most often.