Rating PostsFollow

i am wondering at what point you are able to rate posts for karma. i don't necessarily have a lot to say, but I woudl like to be able to rate up people's posts that are informative and helpful. maybe i am just doing something wrong, though.. any ideas? post limits? or karma ratings of my own or anything? send me a pm if you have any ideas, please. thank you. i se lots of great guides out there i would love to bump on up.