You can get to North Sulawesi by air, sea, and land.
By ship
Manado has a harbour with ships going to and from the more Northern Islands Sangir Talaud area.
Bitung is the major port of North Sulawesi and you can get on board a Pelni ship to sail towards other area's of Indonesia. Some of the larger cruise ships that travel from Australia towards Thailand or other Asian countries also make a stop at Bitung.
By Car Or Bus
You can travel by car or bus from South Sulawesi towards Manado, however due to security issues in Central Sulawesi this is currently not recommended. So, if you plan on travelling by land, check the local situation first!
By plane
Manado has an International airport with numerous flights a week to Singapore, Kuala Lumpur, Davao in the Philippines, and just about all the big cities in Indonesia.
Air Asia, Asia's budget airline now run three flights a week direct from Kuala Lumpur to Manado [Update as of October2010 this Service seems to be terminated/suspended]. Silk Air run direct from Singpore but a quite a lot more expensive.