If this is your kitchen countertop, you don't want to do this. Mainly because of food contact, but also because of wear.
Painting formica can be done, the secret is "XIM Primer". Somewhat expensive, but it sticks exceptionally well to hard surfaces like formica. From there you want to use floor enamels for your finish, or if your doing an "effect finish", use a floor varnish to protect the finish.
This is ok with bath and laundry, but not ok for kitchens. Although, we do on occaision, paint cabinetry that has doors with formica surfaces using this process.