Hi!
Whenever we are dealing with gun rights, we're going to be referring to the 2nd amendment.
To quote it exactly, the 2nd amendment states that:
'A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed'. (Quote is taken directly from the Constitution)
By using this as your backbone, you can make a case for your belief. You may discuss certain parts of it such as 'the right of the people to keep and bear Arms, shall not be infringed'.
You can use this passage to provide support, by directly showing that the Constitution states that the right of the people to possess guns is a right which may not be taken away.
Hopefully, this helps! =)