If you're just biking around campus/Central Sq, you really don't need that many gears. For something affordable, a used single speed/fixie is pretty easy to maintain. If you are going to be living around some parts of Somerville or commuting a farther distance you probably want to spring for something a bit fancier. There is a free and for sale group on facebook(bikes pop up on there often enough), and I think MIT police still does a bike auction in the fall? If you can't wait for that, Cambridge Used Bicycles is where I got my first one, like /u/thefunk3 suggested.
Make sure that all parts of your bike are secured onto the frame! This got my friends and myself a couple of times - people will take your front wheel, the seat, whatever isn't bolted on (i.e. on a quick release). Make sure that when you use your U-lock it is going through both your frame and your front wheel, or use a u-lock and a cable lock to secure everything together. I personally use this, and I find it has just enough room to secure everything(especially with thicker parking meter stems)but I can't recommend anything longer since I've heard people use the extra space to get more torque to break the lock.
From the image provided, it looks like a Kryptonite, Evolution Series 2™
The grey rubber coating is the identifier.
Get a mid-grade Kryptonite U-lock and lock the frame (not wheels) to something solid. If you have quick-release wheels, either replace the skewers with bolted versions and/or use a cable to lock the wheels to your U-lock. Note that smaller/more compact U-locks are safer because they're more difficult to cut or break.
As long as your bike isn't super valuable (> $1000) and it isn't the easiest one to steal in the vicinity, this should make theft really unlikely.
EDIT: Also I recommend replacing your seat post clamp with a bolted version if it is quick release. Seat posts and saddles are annoyingly expensive and it takes one second to steal them if you have a quick release clamp.
Is this bike lock too big?
I live in Chicago and just got a bike.
https://primenow.amazon.com/dp/B005YPK8R6?m=A3MB1SWKKQZ6NT&psc=1
Size: (4-Inch x 11.5-Inch) U-Lock