Nikki,
You've received lots of good suggestions so far, and I have a few thoughts as well.
It appears that you have 11 days to work with. Does that include your flight days as well? Even from Toronto, you'll generally arrive in the Europe the day after you depart, and the final day will be your flight home.
I agree with the others that with such a short time frame, sticking to two countries would be a good idea, and my suggestion would be London and Paris (with day trips). You'll need to do some "homework" and decide which part of each city you want to stay in. Most of us here have a preference, and I prefer to stay in the 7th Arr. close to the Eiffel Tower. It may be a bit touristy, but it's a very nice area along Rue Cler and has convenient Metro access so easy to get anywhere in the city.
For travel from London to Paris, the EuroStar is about the easiest method (London St. Pancras to Paris Gare du Nord). You can save a lot of money by purchasing tickets well in advance, but note that the cheapest tickets are non-refundable and non-changeable. Have a look at the excellent Man In Seat 61 website for details on how to book.
Given the fact you're on a "college budget", Hostels would be a good choice. Not only will that be the cheapest accommodation in some expensive cities, but you'll meet others there that may want to go out to clubs in the evening or whatever, so that you don't have to wander around alone. The Hostel staff are usually exceptional in terms of knowing all the "hot" spots in each city. You might have a look at the HI Hostels website. They have properties in many cities worldwide (sometimes several in each city), and I've found they're well run and not "party palaces". Having an HI membership will provide cheaper rates and preferred booking over non-members. For Hostel stays, you'll need to pack along a towel (Microfibre works well), a medium size Padlock for lockers and flip-flops if they have a shared washroom.
For ideas on what to see in the cities you're visiting, you might stop by your local Library or book store and have a look at some guidebooks. The Rick Steves guidebooks are very nicely arranged and updated every year, and I'd recommend having a look at those first. You could also pack along the Pocket London and Pocket Paris books, as they would be excellent resources to have on hand when you're actually in the country.
Regarding your question on airports, I find LGW much easier to use. It's a smaller airport than LHR and therefore much easier to navigate (LHR is VERY large).
For the cheapest flights, have a look at Air Transat. As I recall, they have flights to LGW and from CDG, so that would work well (I've flown that route with them in the past, but didn't check to see if anything has changed). The only problem could be that they only offer flights on some routes one day per week, but if you could fit your schedule to their flights, that might be a way to save some money. Premium Economy seats will provide a more comfortable flight experience (although for a flight of about 7.5 hours, you should be fine in Economy). I would not recommend booking Air Canada since they only fly into LHR. I haven't checked on other flight options out of YYZ.
You may enjoy have a look at the RS London video and RS Paris video, as they might give you some ideas on sightseeing.
Good luck with your planning!