Yes, seriously... this seems to be an area of recent confusion for some expats. Firstly, let me say to the girls, going to the pub is for boys and girls/men and women/dudes and chicks/lads and lasses, not just for the boys. Do not deny yourself the joy of pub-going because you may think or have been told its a "boy thing".
Step 1: Find a pub. I won't waste my time telling you what to look for in a good pub or how to tell if the people inside are "nice" because one man's (or woman's) perfect pub may be another's nightmare. Pubs are like a box of chocolate... I think cherry cordials are gross and will only eat them if very desperate and even then will only bite the chocolate around the cherry grossness, but someone will seek them out and eat them before touching the caramel ones. (Who are these people!?)
Step 2: Do not dress up like you are going to a club or something. In fact, do not dress up at all unless you like pubs where people dress up and if you do, please stop reading my blog immediately. I simply cannot have you as a reader.
Step 3: Walk in confidently. The darting eyes of an "outsider" will give you away immediately.
Step 4(and this seems to be the tough one for some people): Order drinks. And yes, that's drinks with an S on the end. It is generally expected if you either went to the pub with people or met up with people once there that you will buy a round of drinks. Never just buy a drink for yourself. Never. Never.
Now, I understand buying a round has its challenges, namely remembering what everyone wants and carrying the drinks back to your thirsty friends. The first challenge has an easy solution: do the best you can. If you cant remember what someone wanted, get something non-offensive like a Foster's or Peroni maybe. Just get as many drinks as there are people and you should be ok.
If you have more drinks than you can carry, either take trips (no one will steal your drinks off the bar unless you've made a very bad choice in step 1) or yell at your people across and bar and usually someone will come to assist you (although this may be making a huge assumption about your friends!).
Important: Unless you are a trained waitress and/or armed with a tray, never try to carry more than four drinks at a time. Nothing will ruin your pub experience quicker (and make you the center of unwanted attention) faster than dropping drinks on the floor, sending shards of glass and beer all over people's feet and legs. You may get lucky and drop the glasses on a carpeted floor and at least avoid the noise, but do you really want to be the one responsible for the stains on the carpet that will probably be there for all of eternity because no one will bother to properly clean the cheap carpet?
Step 5: Drink tasty beverages, maybe play some darts, listen to the effects of many drinks on an English accent and enjoy yourself.
To find some excellent pubs that you should support becuase they allow people like me to bring their dogs, check out DoggiePubs!