Welcome to the salty side. I would begin with making a list of fish & other critters you would like to keep in the 15. 15 gallons will put a limit to what species you can house but once you make your selections, it will help in deciding the type of sand to use & how to build a rock structure that can provide all the animals with a healthy & secure environment. Some of the best advise I have been given is to read up, read, read, read & when your done reading, read some more. Research the animals & their natural environment, nutrition needs, temperaments, etc. Do some research on the sand, rock, salt mix & the use of RODI water. Build a good maintenance kit, test kit & read up on proper saltwater parameters. Good water circulation in the display is key so make sure to read up on buying the right powerheads for that job. Read up on building a good clean up crew aka CUC. If possible, read up on quarantining fish & the process, it’s something worth getting familiar with. The main key of this hobby is patience & tons of it. These systems take time to build so take it slow & enjoy the process. Have fun with & I will say it again, read, read, research & more reading.