You should apply for ESTA before leaving for the USA by air. It is an online application and normally is instant (unless they require further documents and want you to apply for a visa). It last for 2 years, and is multiple entry by air and up to 90 days per entry.
ESTA for land travel
For land travel it seems you don't need to have ESTA. But I would still get it, as it wouldn't hurt (in case you decide to fly out to Canada).
You fill that out before crossing the land border. You can find an online copy here. Although I would advise you to get one from the border itself. It basically waives you from requiring a visa, and is for land borders.
Eligibility for ESTA or I-94W
The citizens of following countries are eligible:
Andorra, Australia, Austria, Belgium, Brunei, Czech Republic, Denmark,
Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Japan, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta,
Monaco, the Netherlands, New Zealand, Norway, Portugal, Republic of
Korea, San Marino, Singapore, Slovak Republic, Slovenia, Spain,
Sweden, Switzerland, and the United Kingdom
I-94 (Time Limit in USA)
I-94 is what the USA border give you when you enter. It is electronic. You can check your I-94 expiry date (normally 90 days from entry to the USA), after you have entered the USA.
USA to Canada and back to USA
If you leave the USA temporarily to go to Canada and back (up to 30 days) then they will use your old I-94. However since you are going for more than 30 days you may get issued a new one. Also border control on the US land borders are normally more inquisitive than those at the air borders (from own experience, and my US citizen spouse has similar experiences). You should always take proof that you are leaving the USA within your 90 days each time you enter (e.g. flights, etc...).
You may not get readmitted into the USA
The USA may count your Canada trip as part of the USA trip if you plan to return to the USA. The safest bet would be to fly out of Canada back to Norway, then you aren't making a temporary trip from the USA into Canada and back which would mean your short trip to Canada wouldn't count as part of the 90 days.
If you are admitted to the United States under the VWP, you may take a
short trip to Canada, Mexico, or a nearby island and generally be
readmitted to the United States under the VWP for the remainder of the
original 90 days granted upon your initial arrival in the United
States. Therefore, the length of time of your total stay, including
the short trip, must be 90 days or less.
Source: Visa Waiver Program - travel.state.gov