How do you clean aluminum before TIG welding?

It depends on what type of contaminents are present. Usually, a NEW stainless steel wire brush will do the trick.

If there is a large build-up of contaminants, you may consider using a grinding disc or flap disc on an angle grinder, again being sure that they are NEW and clean.

Also, there are chemical solvent cleaners available for the removal of storage oil on aluminium, I can't think of the names, but it should be available at your nearest welding supplier.