ok got all 3 new chainrings on and my casette and chain im in 1st gear and when i get to about 3rd the chain goes really saggy and even worse as i go down, and how do i get the chain to go on the big ring as i had a bash on before and stopped it from going up, i tried messing with the high and low screws and didnt realy do anything except losern the frontmech up a bit, how do i set this up cheers

Everything you ever need to know about setting up your gears.

…And possibly more than you ever want to

i don't think its the chain length, cause ive just put it on the biggest cog at the front and the biggest bog at the back and it reaches spot on no links need taking out or adding, i think its something to do with the tension that my gears are making the chain look like im using spaghetti lol

cog not bog ha

As per nickc has linked.
click on the derailleur on the pic then follow instruction for setting up rear mech.