Made up of a few small alleyways, Golden Gai really does make you feel as if you've stepped back in time a little! The bars are all tiny little establishments, most of which have only a few seats here or there - and some are literally standing room only. I went with 3 other friends when I visited, and it wasn't uncommon for us to step into a bar and be the only 4 patrons that could fit in there! There was something really fun about that though - it often felt like the bars we went into were open just for us!
I would suggest that you need to be aware when entering certain bars of the signage on their doors. Some are for 'regulars' only, and won't take tourists/foreigners in - don't worry, there are still plenty of bars to choose from that will! The other thing is that some of the bars do have a cover charge to get in, before you start drinking. Again, this isn't all of them - but it's just something to keep in mind, as a several hundred yen cover just to get in and sit down can end up cutting into the budget, especially if you're planning on doing what we did, and bar hopping to a few different places to check out the different atmospheres from place to place!
And what to drink, you might ask? A lot of the time if I am at a new place, I'll ask the bartender for their recommendation (おすすめ or "osusume" is a good word to keep in your hip pocket - useful for not only trying a recommended drink at a bar, but foods at restaurants too!)
I think the atmosphere and the charm of Golden Gai is not to be missed if you're in Tokyo and fancy a tipple!
Getting there is quite easy - just a short walk from Shinjuku Station's East exit!