Next month I will fly to New York with United Airlines operated by Air Canada, I have one stop in Toronto, do I need to pick up my luggage and check in again?
This is one ticket. And do I need ETA for traveling to Canada? I already got the ESTA.
However you do need to pass the immigration requirements to transit through Canada. Assuming you have a Dutch passport, you need to get an ETA.
After that, you have to pass American immigration officials (because of pre-clearance at Pearson, but this means you've already done immigration once you arrive in NY !) then security once more.
Phew ! Sounds like a lot, but in practice this shouldn't take very long
According to the Government of Canada website:
I will transit by air through Canada. Do I need an eTA?
Passengers transiting Canada will need an eTA or a visa, regardless of the length of time in Canada.
Travellers whose planes refuel in Canada going to and from the United States, as well as participants in the China Transit Program and Transit Without Visa Program, do not need to have an eTA or a visa to transit Canada.
There are exemptions for citizens of the following countries:
There are also exemptions for people flying on planes that merely refuel in Canada, but that's not the case in your situation. In addition you should note that ETA only becomes mandatory on September 29, 2016, so you might still make it without one.
Most likely not but Canada is rather complicated and every airport is a bit different. Check out https://www.aircanada.com/en/travelinfo/airport/images/yyz.pdf for Toronto. See section on "international to US". Apparently your baggage is checked through but you still must meed Canadian Visa requirements.