You are in the wrong place (this forum is about folks applying for US Citizenship)
You can visit your girlfriend on a tourist visa for a short period of time (months - but I'm not sure). If you say you are going to live with her, the immigration officer will likely deny your entry (but if you say you are only visiting, it's ok -- but you'd probably need proof of your intention to return to Canada (a return air ticket, some proof of commitments (work, school) back in Canada, etc)).
You cannot work or go to school on a tourist visa.
I'm nowhere near experienced (or a lawyer) on these matters, but I'm guessing that a Canadian with a high school education and no reason (by US immigration standards) to be in the states would have a very hard time getting anything other than a tourist visa.
Gettting engaged, or, better still, married, changes the equation completely.
Two things you don't want to do/have happen:
1) Get denied entry to the US. Once you get denied entry to the US, you have to answer "yes" to the "Have you ever been denied entry to the US" question that those of us who deal with the Immigration folks here get a lot. Being "denied entry" is bad (getting deported, by the way, is worse).
2) Lie to US Immigration. Really (really, really) bad things can happen, now and in the future, when you lie to the Immigration folks. Never do that.
(Remember, I am not a lawyer, this is not legal advice, only something that I might consider doing myself -- consider getting legal advice somehow)
Find out how long a tourist visa can last. Find something that requires you to be back in Canada about that far in the future. Get written proof of that commitment. Go to the border and apply for a tourist visa (I have no idea how to do this). Have your proof and be very truthful. Leave the country before the visa ends, making sure to surrender it officially in whatever way is required. Then, wait a bit and repeat.
Don't get denied entry, don't ever lie, don't overstay your visa, get some real legal advice.