Toronto - Quito via Cleveland and Houston, do I check in for every flight?

I am a bit confused... I am an Ecuadorian citizen and I hold a US tourist Visa and a Canadian temporary residency visa...

I bought a ticket from Toronto to Quito on United's website...

My intinerary starts on Toronto goes to Cleveland, after three hours goes to Houston and after 3 hours goes to Ecuador.

Even though I bought the ticket on united the first flight from Toronto to Cleveland is operated by Air Canada Jazz...

My question is: do I have to go through customs in Cleveland or is it enough to do it in Toronto? (I understand you do it at Pearson)...

And also, I how do I check in for my flights? I only got one confirmation number and this takes me to the Air Canada website but is this enough? Do I check in for every flight on that website?

Finally, what about my luggage? Do I need to worry about checking it in, out on every American connection? Or there is no need...