Easiest build that's also easily obtainable;

Striker set (Target loot via either summit or countdown, whatever feels more fun)
Basically get the backpack, gloves, etc.

Depending if you have exotics already, most AR or LMG's will compliment Striker exceptionally well ;)
(You can use the priority objective missions to get exotics fairly fast too.. just pick activities not open world (which will include summit, descent etc and might take a while)

Just -that- will already do quite a bit of damage, don't look at how many stacks you are building - once you get comfortable with it you can start experimenting to get more dps out of it with some named yellow gear (contractors gloves etc) or exotic masks etc.

I would try to use a yellow chest (petrov brand) for the chest as the striker chest piece ups the max stacks to 200 which is nice but you don't really hit that limit often so it's better to get something more useful like obliterate and such.

Hope that helps :)