So, here's the thing: i am on a situation in which i can't go to the gym. However, I Still want / need to do some pull ups. How can i do it at home without having to buy anything? I've already tried Doing it on a platform above my stairs, but it doesn't give me a good grip.
I've had good luck with the Iron Gym Pull Up bar. It does require a standard-size door with a reasonably secure/level lintel and a few inches room on either side (much to my chagrin, I only have one door in the house where this applies, a bathroom). It costs about $25-30 in stores, but they show up fairly often on Freecycle for free, or on Craigslist for $5 or so. It doesn't look like it should hold you up, but it does do the job. Before that, I alternated using a support I-beam in my basement (the lack of a good grip was an issue there, although it probably did good things for building up finger strength) and heading down the block to the playground (not bad, but it's outdoors, and the little kids look at you funny).
If you don't have anything to use that will give you a good grip, you'll need to buy/steal/borrow something. You can build your own for roughly $20.
You can also spend $25 and get a cheap pair of rings (nice ones will set you back ~$40-$50). With those you can do levers, dips, pullups, muscle ups, shoot throughs, and much else.
If it must be at home, then it really depends on the architecture and furnishings of your house. My friend's place has a floating beam in the living room, which is not bad for doing pull ups on. I don't have such a beam - not everyone is going to have something at home that's satisfactory for pull ups.
So that's when you need to check out places outside of home. Playgrounds, alleyways, forests - get creative and you'll figure something out. Though be prepared to look a little silly!
I've just bought myself a pull up/dip tower, and it's been a brilliant investment. I've done more pull ups in the past few weeks than I have in years. As nice as it is to not spend money, you ought to invest if you're serious about exercise.