Prior to beginning any new program, some find it useful to establish a baseline, which essentially means to take stock of your current status in order to have a point in time with which to compare at some point in the future.
A baseline can be formed across many metrics including body composition, appearance, eating habits, and bloodwork, among others.
In order to establish a body composition baseline, you can schedule yourself for one of the more sophisticated tracking measures such as a Dexa scan, or you can go the simpler route by calculating hip/waist ratio or using a skin fold caliper. This choice depends primarily on the degree of precision required to assess whether you've moved towards your goals - if you're hoping merely to lean out a bit a Dexa scan may be required to distinguish between fat losses and muscle gains. If you're seeking major change, the simpler methods may be sufficient.
If you're primarily interested in changes in your appearance, photos can be very helpful. I suggest choosing an outfit you're comfortable with viewing yourself, and potentially sharing with others if you're ever interested in showing off your progress. A swimsuit or sports bra/shorts combination is the typical attire for such photos. Take a front view, side view and back view photo, then store them away for future reference.
Prior to taking on a change in eating habits, I think it's very important to establish what you're doing currently. Learn how much you're eating overall, how much protein, carbs and fat you're eating, and whether you're consistent or inconsistent in your intake. From this baseline, a few key points typically emerge as the targets for initial change - eating less overall, eating more protein, or eating more fiber, as typical examples.
Bloodwork is another useful tool for establishing a baseline. Standard bloodwork can give your doctor an opportunity to help you identify any current health concerns including markers of diabetes or heart disease, or to help identify deficiencies in key health markers such as Vitamin D or thyroid function. In addition, bloodwork can give you a point of comparison to determine whether changes you make to your nutrition habits are positively or negatively impacting your blood markers, particularly when you're challenging conventional wisdom by doing things like increasing your fat intake, or eating more than the USDA's recommendation of only 0.5g of protein per pound of bodyweight, or the standard 2,000 calories per day.