Well, today I drank beer with a couple of my friends.

"friend" and "my friends" both work

I'm not used to drinking on sSundays, and I don't want to have a hangover/be hungover the next day.

You can say, "I don't want to have a hangover" or "I don't want to be hungover the next day."

SEspecially because at my age a hangover/being hungover is the worst thing that can happen.

Some days I have had to suffer headaches for the whole day.

I felt sick and I didn't want to eat or drink anything either.

I could drink a lot with my friends and at the next day, I waoke up as if nothing had happened the daynight before.

Theose were the good timeold times/the good days.

You can say "those were the good old times" or "good old days" to get your meaning across.

Nowadays, I keep remind mindyself that I shouldn't drink more than three pints of beer unless I want to regret the following morning.

I'm not used to drinking on sSundays, because I don't want to have a hungover the next day.

You could also say “I don’t usually drink on Sundays because...”

SEspecially because at my age a huangover is the worst that can happen.

I remember the days when I was atin college.

I could drink a lot with my friends and at the next day I would wake up as if nothing had happened the day before.

Theose were good times.

Nowadays, I keep in mind that I shouldn't drink more than three pints of beer unless I want to regret it the following morning.