first of all, laughed my ass off at the comments about planting big trees. so love it when people think thats what tree planters do.
Secondly, it's definitely getting pretty late, but if you're lucky, you might still get a job. Start sending out resume's and applications on a huge scale. Because of the state of the industry at the moment, most western companies are not looking to hire too many rookies, so your chances are far better in Ontario. Big companies that usually hire a lot of rookies are Outland, A&M, and Brinkman. Personally I have worked for Heritage Reforestation and will be doing so again this summer. I know that they are currently in the process of hiring right now, so I'd recommend applying ASAP... like the moment you're done reading this APPLY. As was mentioned, replant.ca is an AMAZING tool, it has all the info you'll basically ever need to know about treeplanting, it also lists a lot of companies.
You're going to want an employer that has a bit of a reputation, planting is always a shitshow, no matter who you work for, but you don't want a company that will seriously fuck you over. Don't do anything that sounds really sketchy to you- (being asked to give them money to sign up... seriously, i had that once... i think they thought it was some joyful summer experience program i was signing up for or something).
Ask about camp costs, they shouldnt be asking for more than $25 a day really, and you don't want to be making much less than 8 cents a tree either.
Everyone is different in their first year, the key in your first few weeks (average learning curve is 3 weeks) is to NOT GET DEPRESSED, it's going to be insanely taxing on you both physically, and especially mentally. The money will come though, as long as you stick it out and try your best. However, it's definitely something you should look to do more than one summer, because thats what makes it really worth it- you'll make way more money in your veteran years. I made $4,000 planting in my rookie season, but the planting aspect only lasted one month (our company did other brush work as well which I went on to do).
Most important things to bring- GOOD workboots, DO NOT GO CHEAP on them, and also break them in before you leave. A solid tent, I'd prefer one thats a good size, remember you're going to be living in it. A tarp for over the tent, no tent is perfect, especially after two months outside. BUGSPRAY! DUCT TAPE! pants, that arent jeans (you're gonna get soaked, and jeans are terrible when wet)... the list could go ever on, there's tons more on replant.ca... if you manage to get a job, feel free to pm me and I could give you more details.
So yeah, thats my novel on some basics of treeplanting. Hope you get a job, it's the most mentally and physically overwhelming thing I have ever done, and I can hardly wait to get back at it!!