Hokkaido is Japan's largest and northernmost main island, and the distances between its cities are much greater than in most other parts of Japan. The single biggest planning decision for a first visit is deciding how much ground you actually want to cover, rather than trying to apply a Tokyo- or Kyoto-style itinerary to a much larger area.
Sapporo works well as a first base
Sapporo is Hokkaido's largest city and its main transport hub, with the most direct flight and train connections. For a first visit, it makes sense to base yourself there and take day trips or short overnight trips outward, rather than trying to circle the island.
Plan transport differently than in central Japan
Train coverage outside the main cities is less dense than on Honshu, and some scenic areas are more practical to reach by rental car. If you plan to drive, check seasonal road conditions — winter driving in Hokkaido requires experience with snow and ice that not all travelers have. If you prefer trains and buses, build extra travel time into your plans and confirm current schedules, since some rural routes run infrequently.
Weather shapes the trip more than in most of Japan
Hokkaido's seasons are more extreme than central Japan's: winters bring heavy snow and are popular for skiing and snow festivals, while summers are cooler than the rest of the country and popular for hiking and flower fields. Check the season you're traveling in before finalizing an itinerary, since it affects which activities and routes are realistic.
A useful next step
Use the Hokkaido destination hub to think through how many areas you can realistically cover, and confirm current transport schedules directly with the relevant rail or bus operator before booking.