There are several ways to do 5x5. In some readings, such as John McCallum's, start with a weight 100 lbs (or so) below your 5RM and do 5 reps. Next, add 50 lbs and do 5 reps. Then, jump up to your best weight for 3 sets of 5.
Otherwise, you could get warmed up and do 5 sets of 5 with the...