You don't really 'cook' with it: though it can be used in baking, the oil in the seeds is damaged from higher heat. The best way to use it is to first, buy whol flax (it can go rancid quickly if bought already in meal form), then grind a few days' worth into meal, keeping the whole seeds refrigerated or, for longer storage, frozen. Keep the meal in the fridge as well.
It's good sprinkled on oatmeal (after the oatmeal is cooked) and can be added to smoothies.
The taste is rather nutty.
Even better than flax, IMO, is chia seed. The same benefits as flax, like omega-3 fatty acids, minus some of the potential drawbacks of flax (the estrogen-like effects from the lignans in flax). If you do a web search on "flax versus chia" or just "flax chia" you can find some discussions on it.
Also, if you do a search for "flax" on this site, you'll find several threads where we've discussed it.
HTH!