In addition to starting the book knowledge, there are some flying activities you can start working on while a VFR pilot.
Fly as many cross country flights as you can, bonus points visiting airports you have not been to yet. During these flights:
Make use of Flight Following. Practice your radio skills asking for FF while airborne, listening for your call sign and the instructions that follow, switching frequencies between ATC sectors and checking in with the new one.
Practice precise control of the aircraft. Remain within +/- 5° of the desired course and +/- 50 feet of your assigned altitude.
Practice using VORs. Work on how to tune your NAV radio, verify the VOR by the morse code. Practice intercepting the radial and tracking it to the station, then departing the station on a different radial. This involves setting the OBS on your CDI and learning what the needles are telling you. Bonus points if you do a cross country as Departure Airport >> VOR >> VOR >> Destination Airport.
Practice stabilized approaches. As you turn final, is your airplane settled in on a stable and on centerline descent and stays there with your hand off of the yoke? If you're not at this point, get with a CFI and hone this edge. Learn the "numbers" (RPM, pitch, manifold pressure) your airplane likes to be at to achieve a desired descent rate and airspeed.
If you can gain proficiency in these items before you start formal training, it will be time well spent and make it easier for your CFI-I to teach you the "bigger tasks" of IFR flying.