building my first kayak and my problem is I cannot get 18 foot material for my kayak. so do I buy a 16 foot material or get 9 and 10 foot material, also were is the best place to splice bow stem or at cockpit,or does it matter were.

Lots of kayaks and canoes are built with less-than-full-length strips; some builders actually prefer shorter strips as they are easier to maneuver in the shop.
A search here with the terms 'scarf' or 'scarf jig' will turn up some ideas.
As long as the scarfs/splices are 'staggered' - not close together in one small area of the hull - you shouldn't have much problem. I try to avoid scarfs where the strips are twisting a lot, so usually avoid the ends and cockpit, but it depends on the boat shape.
You can scarf the strips 'as you go' on the boat forms, though I usually try to have 4 full-length strips as 'starters' along the sheer lines - those I scarf on the bench.
9/10 footers or 8/12 footers will do OK - whatever is easy to get. Most strips aren't full length, so as long as you make sure that all your strips are the exact same 'width' you will be able to use your cutoffs as you go.