Particles は＆が

は＆が have the same definition, but they are not the same.
は emphasizes what comes before it (talks about something)
が emphasizes what comes after it (identifies something)

Ex:
私はKirstenです。＜ーPuts emphasis on the name “Kirsten”. I am Kirsten.
私がKirstenです。＜ー Puts emphasis on “I”. I am Kirsten.