Yes, use marine salt for a brackish tank..be sure to add it slowly as going to fast can be hard on/kill your fish. Do you have a salinity tester? If not get on ASAP...if your not sure it's a tube type thing with an arm in it, you dip it in the water and then set it on a flat surface the arm raises to the salinity of your water. (there are other types, thats just what I use). This is a must have for a brackish tank or you'll never know if your at the right salinity for your fish.
Do you know what SG your fish need? If not I suggest you search for that (or ask on here in case someone else can help you with your perticular fish) best choice would be to do both.
Once you know what you need your tank to be at slowly raise the salinity in it (I think it was .002/week) I may be wrong on that it's been a while since I started the process, anyway once you get it to your desiered SG add the salt with each water change(you will learn your proper amount as you do it), but when toping off tank just add fresh water to it. Alway's check your SG along with other perimiters and adjust accordingly.
Have you checked out the post for FAQ here in the brackish forum, it may be able to help you with more and I suggest you do that.
Good luck, really it's not that hard to maintain just a little intemidating at first!