First time playing through WoW and have reached lvl 75. What level do i need to be on to go on raids? and how do i get in one?

You raid at 60, 70, 80 and 85.

Look in trade chat or use the raid browser when you get to 80 to do Naxx.

You may be able to pug a Kara (level 70) or AQ (level 60) raid, as some people may be doing these for the achievement.

Once you get to 85 you can solo a lot of the lower level raids.

There are several different levels of raids, the lowest of which require level 60+, the next require 70+, 80+ and 85. if you are looking into doing some of the lower level raids simply ask around in chat in a main city and you may find a group willing to go.

once you hit 85 and are planning on trying the level 85 raids there are a couple things i would recommend. in order to access the first tier of 85 raids (Blackwing Descent, Bastion of Twilight, Throne of the Four Winds) I would recommend having your character in full heroic gear (item lvl 346) with at least some basic enchants and gems. To start on the most current raid (Firelands) i would recommend having an average item level off approximately 360 with the proper enchants and gems. At this point you can either ask around in trade chat for groups or ask your guildmates.

Good Luck and have fun!