I use Instant Ocean salt.
You'll need to get a hydrometer to measure your salt. They are pretty cheap on ebay. Your dragon goby, according to my research - I have a dragon goby as well - would do well in brackish water of 1.005 to 1.017 salinity. You will need to add enough salt to reach this and you can measure the salinity with your hydrometer. It floats and you read the line at the water level as to what the salinity is.
To do this, and this is how I do it. I mix a 5 gallon bucket with warm water near the preferred temp range (76-78 for a dragon goby), add 1 tablespoon of instant ocean salt per gallon. Stir with a spoon really good so it dissolves. I add my water conditioner. I let it bubble with an air stone a little while (usually while I am draining water into my dirty bucket during weekly water changes), you don't have to bubble it, though. Then pour it into the tank.
Your dragon goby will also need a sand substrate. They like the sand and they suck it into their mouths to get small particles of food. I had gravel in the past before I had sand and he did fine with it, but you should use sand. I am sure he likes it better! They also like to hide so making him a cave or something along side the glass he can hide in would be good.
Mine eats flake, brine shrimp, anything he will eat it. He loves shrimp pellets which is what I mostly feed him and some algae pellets sometimes, but he seems to like shrimp pellets better. He will eat peas. I feed bloodworms once a week and he likes those, too. He's not picky at all.
You really don't have to do anything but add the right amount of salt to your tank for brackish. I added my tips for dragon goby. I would also not put anything right in front along the glass. Mine likes to come out at night - nocturnal and swim around the sides at the top and the bottom, so be aware of that. He'll also pull out plants sometimes. He leaves everything else alone, but he is big so does leave some floating in the morning. He also loves to lay on my bubbler air stone, must be like a massage.
Be careful with the figure 8. I had two of them and they were babies and still became little fin nippers and I had to separate them. They may pick on him. I don't think they do it on purpose. They seem to see something dark, think it's a bug or snail, and peck it.
For brackish plants:
anubias - barteri does better
echinodorus tenellus - pygmy chain sword plant
cabomba aquatica - fanwort or giant cabomba
eleocharis acicularis
hygrophila stricta, siamensis, corymbosa - temple plant, starhorn, compact, giant
myriophyllum spicatum
nymphoides sp. - banana plant
crinum thaianum - onion plant
cladophora aegagrophila - moss balls - marimo moss balls
lemna minor - duckweed
egeria densa - anacharis densa
ceratophyllum demersum - hornwart
vallisneria sp. americana, gigantea, spiralis - jungel val
ceratopteris thallictroides - water sprite
sagitataria sp. - subbulata, perennial rhizome, horizontal creeper
java fern
java moss
cryptocoryne sp - ciliate and becketti, beckett's water trumpet
samolus valerandi
red mangrove
crinum calamistratum, pedunculatum
lilaeopsis brasiliensis - brazilian micro sword