Sin,
It depends on what brand of sea salt you got. Look at the instructions, I know for Instant ocean its 1/2 cup per gallon which means if you have a 25 gallon, then you would probably add 12 cups -- of sea salt and then test w/hydrometer. salinity should be between 1.020 - 1.026 *(high end!!!) I would aim for a 1.024 or so.
You stated you have the water in the tank already? Well, since your tank is new I guess it wouldn't harm mixing the salt in the tank-- but you risk having chunks of salt dissolved and not dissolved in some areas ---- so mix well. Ideally you would want to get a clean trash container or so (clean- brand new) and mix your water in that before placing it in the tank. In the future when you get your live fishes in your tank - just wanted to call it to you attention that it is not recommended you add salt directly to the tank because the dose of salt can damage your livestock ( Hypernatremia --lol for medical people)
add little then test w/hydrometer when you near your 12th cup or so, measure, this way you can always add more salt without wasting $$$$
ALso, did you get anything to remove Ammonia, chlorine from the water you will be cycling your tank with? I'm guessing you used tap water?
What kind of filtration are you thinking of? a power head just circulates the water but are you thinking of getting live rocks$$$$, protein skimmer ????
These are things that I learned but if anyone else has anything to add please feel free too..
You should also check out the thread for starting a tank.. search this site :happyfish