Start with the season: June to October for dry-season viewing and river crossings, January to February for calving, or the greener, quieter months in between. Match the number of parks to the number of days rather than trying to cover everything; three or four parks in a week is a realistic pace on the northern circuit.
Then verify the essentials in writing: which park fees are included, the accommodation category, vehicle type, and whether airport transfers and internal flights are covered. Confirm visa and insurance requirements with official sources, and check recent independent reviews of any operator you consider. This site does not recommend or rank specific companies.