Pesky gears!

Hiya.
I've had my bike int 2 different shops to get my gears fixed and each time it comes back with something different wrong with it, so I'm trying to learn how to do it myself.
This time round my rear derailleur seems to lag when I'm on the trail which is knackering and annoying!. So when I'm in say 3rd and I want to change up to 5th I flick the switch and nothing happens. About half a min to a min later it'll all of a sudden click into place from 3rd to 5th which is dificult as by then I might be going uphill!. Is this jst a simple adustment on the cable tensioner by the shifter leavers do you think or am I gong to have to start twisting screws ect.
I've had my bike int 2 different shops to get my gears fixed and each time it comes back with something different wrong with it, so I'm trying to learn how to do it myself.
This time round my rear derailleur seems to lag when I'm on the trail which is knackering and annoying!. So when I'm in say 3rd and I want to change up to 5th I flick the switch and nothing happens. About half a min to a min later it'll all of a sudden click into place from 3rd to 5th which is dificult as by then I might be going uphill!. Is this jst a simple adustment on the cable tensioner by the shifter leavers do you think or am I gong to have to start twisting screws ect.
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loads of info on Parktools re set up etc.
Parktools :?:SheldonBrown
pinched it.
Parktools :?:SheldonBrown