I'm going off of this build.
http://hellgate.ingame.de/skilltree/marksman/?l=50&s=030721070007020000070300a00

how many att. points should I put into each? And, what skills should I use for my left/right click and which should I use for my numbers (1,2,3,4,5... etc...)

att points keep for gear, you'll need it. as for how you should lay out your skills, it's up to you - personal preference.

ya but like, per level how much should I put into accuracy, stamina, and willpower? and should I ignore strength?

depends on how you want to play, strenght will grant you better armor options which will takea you towards a more defensive build etc

Like the guy above said put your stat points acording to your gear, if you're unsure you can always save a few poiint, your char is still functional.