You can fill up air on a gas station when it's low. Most gas stations. You can also check your air pressure with a gauge. You can buy it in Walmart or any other stores that sell auto stuff. You can also borrow pressure gauge at some gas stations.

In the future I'd always take a car to the same place. If you take it to new places that don't know you, they might "find" stuff that you "need" to fix.