Born in Chaos

http://www.torhead.com/skill-calc#400bsrbdMsRtzZbcMZh.1

When I was leveling I used this build. Right off the bat go straight into marksman, don't worry about all the other trees just yet. Start with steady shot, your snipe/ambush are your main abilities while leveling, I usually would start any fight with Ambush with a followup from Snipe.
As you go up the tree be sure and pick up Heavy Shot and Precision Ambush, if you can time things right you can knock back with your cover pulse and as they are running back use ambush and shoot them back again, it's a great way to run things if you ever pull aggro off Kaliyo.
Snap shot - a MUST. Ambush then Snipe = quick kills.
Followthrough - same combo as before just add a followthrough to the end of it
Rapid fire - I personally got this ability although I didn't normally use it you may want to use this point elsewhere as in Deadly Directive in the Lethality tree.
Now that you are level 40 and have all the talents in the Marksmanship tree, you can start to worry about the others.
Snag Energy Tanks and Gearhead first just to give you that extra energy / damage
Vital Regulators - great for fighting elites in case you start to get low, stun and pop your Adren Probe

Let me know if this guide worked out for you. Make some tweaks, play the way you want to play; this is just a guide on how I did it.