M4s are the most popular rifles, so there's a lot to choose from, with a huge price range.
What's your price range for a rifle? That'll also effect which brands you can afford, haha.
The "go-to" starter M4 seems to be the King Arms M4. I believe it's around $300.
If you're in Toronto, there's plenty of walk-in stores that you can visit.
After you've got a rifle, you may want to set aside another $100 for some accessories, like a sling, a foregrip, and an optic.
You'll also need a good battery, and a good battery charger, which should run you less than $100.
If you go AEG (electric), then you can get magazines for pretty cheap. A box of plastic KA magazines can be had for something like $40.
You'll also want to put aside another $150-$200 for face protection, vest, boots, and maybe a uniform.
Note that once you buy a gun, you may need to downgrade the spring, if it's shooting the BB too fast. Most outdoor fields limit you to 400 feet per second, and indoor is generally 350 FPS.
As far as gas vs electric - most would recommend that you start off with an electric.
I find that gas is a lot simpler than electric, and if you've got the money, pick up something like a KJW M4, which is pretty much ready to go out of the box, with the exception of a velocity reducer. But there are a lot of draw backs to using gas over electric, and you're probably better off with an AEG, and playing a few games, until you can get a feel for the differences.
For outdoor environments, there aren't too many in the GTA. As you step outside the GTA (an hour or more outside Toronto), there's several fields that people will organize games on. Keep an eye on the Ontario events subforum. Event organizers will post about upcoming events in there: http://www.airsoftcanada.com/forumdisplay.php?f=68