As designed, J/24s have a very neutral, maybe slightly lee helm. Tuning for racing, within the rules, consists of
1) making the mast as short as possible. It's measured from the hounds (top of headstay pin) down, so it gets shortened by cutting the bottom. Masts are expensive, so you really want to be careful. At a given headstay length, it will then rake further aft.
2) Make the headstay as long as possible within the rules. That rakes the mast and entire rig back.
3) allow the mast to be as far aft as possible in the partners. Check rules.
4) Set the position of the butt on the beam in the forepeak to achieve the desired prebend in the mast according to the mainsail manufacturer. Typically in the range of 2 inches.
5) Fair the keel within the rules to the maximum forward position. This is a much bigger deal.
The point of it all is to make the center of effort of the sailplan as far aft as possible and the center of lateral resistance of the keel as far forward as possible, to give you some weather helm.