I've just bought a new 18-gear mountain bike and I don't think it's been set up very well. The gears are loose even when going on slight downhill (no extra speed built up when pedalling) and I can't get the left gear shift to go any further than halfway round when I try to get onto a higher gear. I've tried to tighten the cables but I really don't know what I'm doing. Can anyone offer some advice to a clueless commuter?