Hi @jacie,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The process will depend on the condition of the shed. If you had some photos you could share, this would certainly help. Let me know if you need a hand uploading images, I'd be happy to help if required.
You'll have to start by giving the garden shed a good clean with sugar soap to remove any contaminants that will affect adhesion.
At this point, if the metal is in good condition, you will then move on to priming and painting. However, if there is rust, there are additional steps that I'd need to see the actual shed to assist with.
Once clean and dry and any rust has been treated, you'll need to apply a coat of primer. My suggestion is White Knight Rust Guard Grey Quick Dry True Bite Primer Paint. This can be done with a paint brush, roller or paint sprayer.
You can then use two coats of any exterior paint such as Dulux Weathershield.
Check out How do you paint an old shed for some further guidance.
Let me know if you have any further questions.
Jacob
