Please learn to use the search function - most basic issues, in one way or another, have been discussed at least once.
But I want to welcome you and not scare ya off, so today gets you a freebie. It's likely your IAC (Idle Air Control). You can clean it by spraying it with brake cleaner, but on a 5 year old Stang, it may have seen it's better days...it's fairly cheap, last I recall.
As stated, do a seach of this forum for "idle die iac" and you'll find more details.
And again, WELCOME to StangNet!!!