MIT does not use the Common Application. Instead, we use our own system, which lives at my.mit.edu and goes by the name of MyMIT.
The 2018 application will be available in August. You may create a MyMIT account even if you are not sure you will apply. In fact, it's probably a good idea to do so especially if you aren't sure!
For those of you for whom college applications are still far distant, creating a MyMIT account will subscribe you to the monthly admissions newsletter of awesome stuff going on at MIT.
And as the application season approaches for you, we will occasionally email all applicants to gently remind them of upcoming dates, deadlines, tips, and so forth.
MyMIT is also where you will get the information for your alumni interviewer since at MIT you contact your interviewer while you are applying.
And then, if you do decide to apply, you will eventually do so through MyMIT, which also contains the entire online application.
So just head over to my.mit.edu and have at it!