In short, a low-end brackish tank (up to 1.008 SG) is not much different than a freshwater tank (which is 1.000 SG). The only differences you'll notice are that most plants don't survive as well in brackish water (java moss and java fern seem to do fine), and that it's a little more work to do water changes on a brackish tank (as you have to premix the water and salt before putting it in the tank). Brackish tanks also use Marine salt (not aquarium salt) in order to raise the salinity (or amount of salt in the water). Marine salt provides a lot more trace elements and minerals than aquarium salt, so you have to use marine salt (the best for its cost seems to be Instant Ocean salt mix - not the reef crystals one).
Don't just jump right into brackish, though. Make sure you read up on it, as raising your salt level too quickly will kill the bacteria in your tank. It's easy to avoid that happening, as you just need to make sure you don't raise the salinity by more than 0.002 SG per week.
You can use a hydrometer (one that reads from 1.000 SG to 1.030 SG) or a refractometer (more expensive) to measure the amount of salt in the water.
