1)Get a pull-up bar (those you put on the doorframe);
2)Put it on a doorframe you go through often (such as your room's);
3)EVERY SINGLE TIME you go under that door (I mean
EVERY SINGLE TIME, even if you have your backpack on, even if it's 2 in the morning... okay, be reasonable, but you get my point) try to do 1 pull-up;
4)Every MONDAY, WEDNESDAY and FRIDAY, do 5x MAX EFFORT PULL-UPS. That means you will try to do your highest number of reps and really, REALLY put in your max effort (if you can't do 1 pull-up yet, you can still do a max-effort set...). After EACH set, you should be barely feeling your arm (rest about 2 minutes between sets);
(
YES, IT'S SUPPOSE TO BE HARD AND PAINFUL. The idea behind this method (muscle-failure training) is to literally break your muscle-fibers so they rebuild themselves, but this time stronger. That's where the phrase "No Pain, No Gain" comes from.)
You will feel a lot of pain one or two days after each of these 5 max-effort sets. DO NOT refrain from doing one pull-up everytime you go under the door.
Be careful not to hurt yourself. Just because it's painful doesn't mean it's working - use your head and don't hurt yourself
Good luck! This method helped me a lot!