Sapporo, the biggest city and “capital” of Hokkaido island, is likely to be your entrance port from southern Japan. Given the distance, you are probably going to reach it by air.
For more information about this, check this link about how to reach Hokkaido in general.
If you arrived by air, you are now at Shin-Chitose Int’l Airport. From here, you can take the Airport Rapid service of JR Hokkaido, and arrive at Sapporo Station in less than 40 minutes (the price is around 1100 yen).
If you have a Japan Rail Pass and are heading north from Tokyo, you can reach the final stop of Hokkaido Shinkansen, which is Shin-Hakodate-Hokuto (about 4 hours from Tokyo). From here, proceed onto the Super-Hokuto Limited Express train, that will take you to Sapporo in 3 hours and a half.
Be aware that if you want to move at once from Tokyo to Sapporo by rail, it is going to require you at least 8 hours, train change included. Consider it only if you are using a Japan Rail Pass (as it covers the otherwise expensive fare), as taking an airplane for this distance is definitely going to be cheaper and faster!
Navigating the city
Sapporo is easy to get around. Planned and built at the end of 19th century on north american cities’ model, the city is a grid of boulevards and avenues with a simple numbering system. Inside the city there is a network of 3 subway lines, a tram loop line and the JR line with a frequent service.
The hearth of Sapporo is Ōdōri Park, which is also the location of the Snow Festival, located about 1 km south from Sapporo Station. You can take the subway, walk overground or, especially in winter or during rainy weather, in the underground passage, with shops, cafes and restaurants.