Always try for Aquarium safe salt sold at the lfs
. It's actually pretty cheap.
Many people will contend table salt is okay...but why gamble? Some table salt has additives, and I'm not sure what brands.
Same for Rock Salt. Some people swear this is okay, but some rock salt, especially the type sold at Home Depot type stores have caking agents.
A small box of Aquarium Salt by Aquarium Pharmacueticals is relatively cheap and will last a while. At 1 Tbsp per 10 gallons, sounds like you'd only use 3 Tbsp to begin with and you'll only need to replace 1 Tbsp at a time every few water changes.