Depends if they are in fresh water in the shop. If they are, then yes, bring them home, let them settle in for a few weeks, then increse the salinity slowly, start adding salt slowly and measure it with a hydrometer. Put the salt in a bucket with the water as you do a water change and measure how much it reads with a hydrometer, then you can get an idea of how much per bucket.
Eventually it wants to be 1.006, on average but they do benefit from slight salinity changes, as they would naturally have this.
If they are already brackish, ask your shop what level of salinity and match your water accordingly, as i said, they can take a varience in it.
Marine salt needs to be completely dissolved before it is added to the tank, undissolved crystals can be fatal.
Air stones in the buchet with the salt in do this but we found it messy so we dissolve our salt in hot water in the bucket first, then stir it well to airrate it.
They do like bubbles in the tank, airstones are good or venturi device. Water with salt in does bubble higher though, so you need a good condensation cover.
Hope this helps, sorry if i got carried away with the advice before, didn`t mean to make out that you didn`t know anything about fish!! It is just sometimes difficult if it is a fish you have never kept before.
So, don`t worry too much about the salinity levels because they do like it to vary a bit, between 1.004 and 1.008 is usually about right, although i have heard of people keeping them upto nearly full marine.
As tides come in and out the salinity changes so that is why they an tolerate such a variety. The main thing is to increase it slowly.
Clare