You could take one or the other, or both. The difference is in the Omega-3 acids that your body gets. Fish oil has DHA and EPA. Flax seed has ALA, LA, and OA acids (which the body turns into DHA and EPA once in the body). I take both in order to get some variety. If money is an issue, drop the flax and take only fish oil.
How much you take is predicated on the size of the pill. My fish oil pills are 1 gram each. The recommended doseage on the bottle is 1-2 pills per day. You should be able to find doseage instructions on the bottle.
Oh yeah, one other thing. You should double check to make sure that the company has eliminated all the mercury trace in their product. Most reputable companies will take care of that, but the bargain basement brands you never know.