I probably wouldn't since my son was born when I was 24 and I have no idea if this is a divergent timeline or not. But, in the past, I've thought about this kind of situation way too much for a normal person, (but only along the lines of a one-way, Days of Future Past trip to my younger body), so I'll play along. Because my parents redshirted me in kindergarten, I was a sophomore at TCU when I was 20.
1. You can recover from your less-than-stellar freshman year, you just need to learn how to study and how to write. You can best learn the latter by reading as many Bryan Garner books as possible and taking the advice to heart. The former is best achieved by doing your assignments as soon as possible and using the syllabus to get ahead. Use your knowledge of later course work to formulate questions for class and office hours. Learn the Pomodoro technique for regular studying. Yes, that's P-O-M-O-D-O-R-O.
2. Drinking is largely a waste of money, though scotch can be very nice. Don't bring alcohol to girls, especially underage ones, hoping to get some. You don't need it. If she needs it to fuck you, she's not really into you. You won't lose anything by not fucking her. And avoid drunk girls. Having a clean record and conscience in 20 years is worth it. Why? Think Anita Hill and Clarence Thomas but worse.
3. Don't take German and prob & stats with (blonde girl you've known for ever who, unbeknownst to you, is a newly enrolled freshman). Take Chinese and Calculus 3 with your buddy. Her ex transfers in next year and she is unavailable by the time you figure out she might have interest. Plus the study of vectors in Cal 3 sets you up for a better time in prob & stats, mechanics, and electrostatics. Buy Div, Grad, Curl, and All That, since you suck at Cal 3. Take symbolic logic under math, not philosophy to help fill out your hours for the math double major.
4. You're not fat, just a little overweight and out-of-shape. Here's how you cook and eat. (Ten minute digression.) Here's how you work out. (Another 10 minute digression.)
5. Following up on the last issue, you have a disproportionately long torso and wide feet. (Five minutes explaining clothing and shoe sizes and what actually looks good on you.)
6. (Ten minutes explaining various health, physical and mental, issues and maladies.)
7. (Ten minutes naming names.) Your future wife is X. She's at school at Y. She'll get a job at Z school after school.
8. (Ten minutes talking about career and academic interests and goals.)