1) there are 3 main nutrients that almost all nutes will contain in varying proportions to each other- nitrogen(n), which is involved in new branch and leaf growth, phosphorous(p), which is essential for bud development and photosynthesis, and potassium(k), which helps regulate water transport and a million other things. these are the most essential mobile nutrients to marijuana. you will need to adjust these nutrient levels depending on where your plant is in its life cycle (it needs different food depending on what it needs to produce). a lot of the better nutrient lines will include feeding schedules that tell you precisely what to use and when.......
http://foxfarmfertilizer.com/feedingfox.html
2) flushing is one of the most contested aspects of growing as far as i've seen. essentially, unless you really overnute your plant throughout its life or use a fertalizer that facilitates formation of toxic salts there's no reason to flush. i could explain to you in scientific detail why not, but if you really want to know go to the homepage here and search "koijn flushing". that dude knows what he's talking about. otherwise i'd have to go into a long science lesson
3) you didnt ask for it but here's some advice: people will tell you to use organic everything otherwise "you get super bad toxic chemikels in ur roots and stuff." these people dont realize that no matter how that nitrogen you're putting into your soil/hydro setup is broken down into ammonia and is used by the plant to synthesize necessary metabolites no matter what so if someone tries to tell you not to flush or you wont get salt buildup or whatever because you're using organics beware, they're an idiot. the only way to ensure there isn't anything harmful in your grow medium is to use quality products and know what's in the products you use.
this is just a little weed growing experience folded in with quite a few years studying biology. i hope it helps. good growing