Most people when they first hear about 4chan go straight to /b/ and ruin the fun of exploring the other boards. Therefore I will tell you from my experience what to expect from some of the boards.
/b/ - used to be a shit hole for depraved sociopaths that are either fat, pathetic, or just plain stupid. Now it's full of newfags that either don't know what half the stuff discussed there is or full of oldfags that are unwilling to do anything new and interesting. Also expect to see the same troll threads at least 3 times a day. NOTE: FULL OF ASSHOLES
/d/ - If you have a fetish that can be drawn in japanese anime style then go here. from my favorite weight gain threads to Dragons raping cars. Any kind of fetish you can imagine is posted here in anime form.
/gif/ - porn, porn, and more porn except in gif form. Occasionally you may find gif threads that aren't about porn (1 or 2 threads a day).
/h/ - regular Hentai (anime porn). If you're not into dragons raping cars and your only fetishes are big boobs, big butts, or just plain regular pictures of naked anime girls. this is the board for you.
/r/ - The board you go to when you want someone to find something for you. 99% chance what people will give you won't be what you want, will be useless or stupid, or they won't give you anything at all.
/r9k/ - People who use /r9k/ are either: pathetic, boring, have nothing good to say, sad and lonely, all of the above. that being said, on a good day there are very few assholes here.
/new/ - it is supposed to be for news but mostly it's just full of retards voicing racist opinions.
/vp/ - Pokemon board. All things pokemon. Gary Oak is god here and is to some degree a paranormal phenomenon. posts showing Gary Oak will get "gets" much of the time. /vp/ is kinda like every other board all put together but only in terms of pokemon.
/x/ - This place is full of weirdos. I know because i'm one of them. All in all it is a decent board with cool stuff to offer. with many creepy pasta threads, and people streaming various horror movies it's a fun place. Unfortunately at least half of the people there are tripfags (myself included). for those that don't know a tripfag is someone that posts using a name that is secured with a password so no one else can try to pass themselves off as you.
so this is my guide. Keep in mind these are only the threads i've been too more than a few times. If you go to 4chan try to go to other boards other than /b/ it's not as digusting as you think most of the time it's just stupidity. Thank you and keep reading