Thank you so much MapleG! Yes, this is all on one ticket.

When I arrive at the airport in Pearson, do I go to US immigration and customs first? Then do I check in my luggage and get my boarding passes?

Andrew G-you go to Air Canada counters first to check in (as if you are going on a US flight) Then go to US immigration/customs after you have checked in.

Suggest you check in 3 hours prior to departure

Thanks MapleG, much appreciated!

Just to give you the whole story, the order for US-bound flights from Toronto is:

Step 1. Check-in/tag baggage.

Step 2. Clear US immigration and customs.

Step 3. Drop off any checked baggage.

Step 4. Clear security.

Step 5. Enter the gate area (which has the usual restaurants, shops, etc.).

Sweet, thanks for the step by step MattinTO!!

Oh just another thing. In step 1, when I check in my luggage, do I keep it with me instead of giving it in to Air Canada because I need to take it with me to US customs/immigration?

Yes

When I arrive in San Francisco can I just go straight to my gate for my next connecting flight? I won't have to go through customs or security again right?

Yes on the question in #7 -- the Air Canada folks won't take your checked luggage at that point because they know it has to clear US customs.

Yes/probably on the question in #9. Yes you won't deal with customs again, probably you won't deal with security but it depends whether the layout in San Francisco allows you to get from Air Canada's gate to Singapore Airlines' gate within the same secure area. Maybe an SFO expert will know.