get in contact - something you will do:
"I will get in contact with her tomorrow." (If we are doing it now, it's more common to say, "I am contacting her now" rather than "I am getting in contact with her now.")
be in touch (OR stay in touch) - to remain in contact.
"I am in touch with him." (I have his contact information. I know how to reach him.)
"Let's be in touch." (Let's check back with each other soon.)
"Let's stay in touch after you move to Australia." (Please, let's continue to contact each other {by email, phone, etc.} after you move.)