Carrera Vulcan- How do you use the gears? Please help

Hi, I have just bought a bike after 11 years of not cycling and am really confused on how to shift gears. I have 8 gears on the right hand side, 1 is easiest and 8 hardest. On the left there are only 3 gears, again 1 seems to be the easiest. The main area I am confused by is the left hand side gears, in what situation would I use them?
I have been told you should not have these gears in certain combinations as well could anyone please advise what they are? Thanks for any help
I have been told you should not have these gears in certain combinations as well could anyone please advise what they are? Thanks for any help
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You've got the front (left) and rear (right) shifters. The front shifter controls the derailler on the chainset, the 'front' derailler (near the pedals) Most riding will probably be on the middle ring at the front.
The rear shifter controls the rear derailler/mech.
Basically, when you're on the small ring at the front, use the biggest 4/5 cogs at the back, on the middle ring you can pretty much use all cogs on the back, and when on the big ring use the smallest 4/5 cogs on the back.
"As I said last time, it won't happen again."
In general I stick to 2nd(middle) cog on the front for trail riding unless it gets very fast. If I'm out on the road then 3rd is usually a good option for downhill however 1st is the best for steep uphill climbs anywhere given you're in the correct gear.
Basically don't use gears that makes the chain go into the extreme positions and the most diagonal that it can go, its not healthy and theres better alternatives.
http://www.bikeradar.com/forum/viewtopic.php?t=12647117
Parktools :?:SheldonBrown