Going to the bar

Festivals, exhibits, farmer's markets, specialty shops...usually the people there love talking about their craft. Charity or volunteering can be good too. Church, even. Adult education classes. Check your newspaper or local library for all kinds of events, classes and opportunities.

Bars kinda suck for this because you'll encounter a disproportionate amount of people who are toxic in one way or another and you yourself might get dependent on the alcohol component.

Find a hobby in which to immerse yourself instead!

Go to a place that has to do with something you're interest in, like if you're a reader, go to a book store and browse. If you like coffee, go to a coffee house and sit down with a scone, etc...and strike up a conversation with someone.

I think going to a pub or a bar makes you a irresponsible person.

Depends on the other patrons and type of bar. Blue collar type sports bar is good for one guy cause there will be plenty of others like you.

I made a lot of friends when I was a teen and used to go to Metal gigs. At bars or clubs I usually ended up drunk, at a corner crying coz no one wanted to talk to me.
Exhibitions and the beach are nice places to meet people.

Kinda what Wigs said. Bars "can" be toxic but it's not always. I met my buddies from bars but that started because I got introduced by a friend I went to the pub with to her friends.

But, you could go to bars on occassion and do other things suggested. I sometimes go to bars solo because I like to chill out with the atmosphere and I've had a few people come up and chat, so I don't see why wouldn't be the same for you.

Just remember that if you make bar buddies then that's most likely how it'll stay, if you want buddies in general then it's not impossible to find in bars but other avenues may be best. :)

Bars are the last place you should find new people.
Go and do things... Gigs, concerts, galleries, parks.....

It's a start, but, don't make the bar your only social outlet. Wind up being a "bar person" who can't make friends outside of it, and only see the friends there. Check out events in your area, galleries, etc..There's more out there than you think.