First you need a Tungsten inert gas welder (T.I.G. for short).
The second step is to make sure the aluminum is clean. If you just bought it , and it looks clean ; it isn't trust me. Steel and aluminum are totally different in a lot of ways and that is one of them.
I'm going to skip the safety instructions and shopping list , under the presumption that you already know the basics.
Spray the area to be welded with acetone.
rinse with clean water to remove lingering residue.
then use a wire brush that has never been used on anything but Aluminum to clean the area.
I'd go with a 4043 welding rod , it's a popular choice.
Make sure to use clamps so that the 2 pieces of metal are squeezed tightly together with no gaps. (very important)
I like to preheat my metal even if it's thin sheets , seems to come out better although it isn't usually called for in specs.
side note ; If you have to step away from your work for more than 8 hours , you're going to need to re-prep it since it will have formed a layer of Aluminum oxide by that time.