I would start by reading the instructions and filling out the medical application. It's officially on line now, but you can find PDFs of the old form the online application is based on as well.
There are three practical sections:

1. Question 18 (a-w) which ask "Have you ever ..." If you answer YES to any of these, you need to consider things further.
2. Visits to a physician, etc.. for the past three years. Again, anything other than a regular checkup needs to be investigated.
3. Any drugs you are taking. You can get an idea of what drugs are going to be an issue by looking at the member section of the AOPA website.

In addition, you'll get normal vision, "hearing", and bp checks. Those need to be acceptible (20/20 w/correction in each eye, colorblindness test, normal conversational words, and below about 145). In addition, the doctor is obliged under the current rules to inquire about sleep apnea risks, especially if you're obese.

If you've got YES answers (or drugs or dr. visits), one way to preserve your options is to print the application (either the online one or just fill out the paper one) and take it to the AME for a "consultation." Do not give him the control number from the online application which he needs to start an official examination. Once he starts an official examination, if he finds a problem he's obliged to defer/deny you and that's going to lock you out of things like flying Sport Pilot.

There's very little difference between the second and third (the third only requires 20/40) vision, unless you have one of the special conditions.