Welcome to Mac and I promise you're going to have a great time though everyone has their own adjustment period and yours may or may not be longer/more intense than your peers.
The most important thing to remember about any social situation is:
A. Never give up
B. Never sell yourself short
For instance, if you start talking to a group of people who aren't very welcoming, don't get discouraged, try again with another group or perhaps just one person at a time. Don't sell yourself short (now or ever) by thinking that one or two or several for some of us, bad social experiences define us as people or that we'll "never fit in".
That's a wonderful thing about university.
With over 20000 undergraduates, you're sure to find a crowd that you can be yourself around and feel comfortable with; it just takes some time.
Maybe bonding over electrons and/or mitochondria isn't your thing - talking over coffee or in the locker rooms after a big game might be.
Or perhaps you're really a bit shy and can't bring yourself to dress up all in maroon like the Maroons (though I hear that's an amazing way to meet new people), so chatting up one or two people in tutorials, labs, near by residences, etc might be.
Just keep trying - every person you meet is a new possibility.
Do give yourself a bit of a push to get started but don't feel forced to jump out of your skin and be somebody you feel pressured to be.
If bonding over annual physicals or clubbing at 1280 make you feel downright uncomfortable, you don't have to do that.
You will find friends, you will meet people, you will adjust, these will be some amazing years, you will be accepted and you will begin some amazing adventures.
You can always start again tomorrow
Nothing (except those wall carvings) is set in stone here