first take some soapy water and apply it to your tire to see if it has any punctures in it. if it does then it will bubble. if no bubbles then take your valve stem cap off and see if it bubbles.

things you need to replace it are
valve core tool-$2
valve cores-$1

what to do
if you want jack it up
let all the air out of your tire
remove the core with the tool
put the new one in
air it up