To change a tranny filter, you will need the gasket and filter, they usually come in a set, and about five quarts of transmission fluid. Hopefully the transmission pan will be easily accessible, most GM cars have 2 screws that hold the filter on after you have dropped the pan. Clean the pan before putting it back on, checking for metal shavings in the sludge(this is a sign of worn out clutches). In my opinion the transmission should also be back flushed, but that is a shop function, but it does increase the performance and life of the transmission.