Tighten chain?

[Radius]

Seeing as my chain seems to come off far too much when changing gears, and when (if) I fall off (only happened a couple of times, and I fell on the other side), I'd imagine it could do with being a little tighter. Logic tells me I need to adjust the rear mech in some way, perhaps the pulley bit that hangs lowest down?
Had a search around but nothing says anything about that...how should I do it?

 

[John the Monkey]

How to calculate Correct Chain Length

Bear in mind that it may not be the chain length at all, but could be the upper/lower limit screws on the derailleurs.

Adjusting a rear Derailleur
Adjusting a Front Derailleir

 

[Radius]

I have recently adjusted myself, and had adjusted my front and rear limiters, so I don't think it's that, but when I first got my bike, I took it in to the shop due to a clicking sound, and said it was very hard to change gear. His reply was "I've loosened the chain a bit", so I assumed that's what I had to change, but he didn't mention putting new links in...

 

[ColinJ]

I have recently adjusted myself, and had adjusted my front and rear limiters, so I don't think it's that, but when I first got my bike, I took it in to the shop due to a clicking sound, and said it was very hard to change gear. His reply was "I've loosened the chain a bit", so I assumed that's what I had to change, but he didn't mention putting new links in...

It's possible to get stiff links in a chain. He probably just freed up one or more of them.