If you can, lager...
- Make a starter according to
http://www.mrmalty.com/calc/calc.html be sure to set the parameters correctly you'd need 3.6 vials of lager yeast for a 5 gal, 1050 beer. Or, you'd need 2 vials in a 3L starter. Since you're new, consider buying the 3 vials (you could get by with 3 but may want to go for 4...up to you).
- brew it...
- chill it to as close to 50 degrees as you can get it without taking too long. If it's 40 degrees outside, then your groundwater is ~ 32 degrees? Have a chiller? Snowbanks? Try to cover the wort while chilling if you don't have a chiller (many cover while usign a chiller too). I have a 3/8", 25' copper coil as a chiller. My groundwater is ~45 degrees. I can chill 5 gal from 212 to 70 degrees in 15 minutes (or so). I (very gently) get a whirlpool going using my spoon. Be careful not to agitate/aerate the wort, but moving the wort past the chiller will chill it down faster.
- drain to your sanitized (I like StarSan) fermenter, cover with a sanitized cover and chill further to ~50 degrees.
- pitch your yeast and cover with a sanitized bung with blow-off (preferred due to aggressive fermentation)
- I rarely ever 2ndary... I'd let it ferment in the primary for a month, rack to the keg, put it in the fridge at 32 degrees and let it sit for another month. Force carbonate at your required PSI and enjoy after the requisite amount of time has passed to carbonate.
Now, you mentioned the garage being ~50 degrees... is it stable? Guessing not...temperature fluctuations are bad. Couple options, do you have a large tub of water to put the fermenter into? The larger water mass will reduce temp fluctuations from day to night.
If you can't get stable, 50 degree storage, consider doing it as a yeast and ferment inside in a stable environment.