I went through a similar experience by switching from part-time to full-time. The change is a major mind shift for me as a startup founder to operate with speed and focus. Personally, I don't believe a startup can be successful if the founders cannot jump in as full-time.
In order to prepare for a full-time transition, I recommend you to look into the following aspects:
1) Psychological readiness: Startup life is a rolling coaster experience. At an early stage, there is a lot of pressure with little light in the tunnel.
2) Financial readiness: You may not be able to bring home much income at the early stage of a company. I recommend you get yourself prepared to deal with the financial pressure by saving more before you quit the full-time job.
3) Family readiness: Startup founders usually don't have a life in the early statge, there is no 9-5 routine. It will be easier for a single individual to make a switch. But if you have a family, you need to make sure that your family members are on board.
I can share more details on my experience and tips on how to deal with the transition. Good luck to you!