Yup pull ups are very difficult. And sadly like running the only way to train for them is to do them.
You are probably finding them more difficult because you are fat and weak, this is to be expected.
Get yourself nto something you can hang off and do one, then two, and as many as you can do in a day when you have a spare minute. You will find over time the weakness will leave you and you shall grow hard and unto the world of men.
The inner knowledge of this is difficult to comprehend and is only passed on orally. You should meditate on this well.
This could well get shot down in flames by those more qualified to answer, but here goes....
Ultimately the above are correct, just keep working on them. However, some gains may be seen if you spend some time concentrating on the different muscle groups used in a pull up. The specifics vary depending on the type (under-arm grip/ over-arm grip/ closed/open etc.) but generally it's your upper back/ lats and your biceps.
If you've access to a gym or even just some weights, then with the help of google you should be able to come up with a range of exercises to help you. Biceps is obvious, for your back try seated or bent-over rows and lat pull-downs.
If you take a break from the pull-ups and work on these muscles you should notice a marked improvement when you come back to them.
As a side note, check your technique is correct. It's not exactly complex gymnastics but it can make a difference. Again, google will turn up plenty of preachers on this topic.
Just noticed you've left yourself two days to crack this, so it's a little late for training. On that note.... technique. An extra couple of pull-ups should be achievable by ensuring you're pulling evenly with all muscles used. As said previously, back and biceps share most of the load, so if it is, for instance, your arms which you feel giving up first then really squeeze with you back, or vice versa.
Believe me, under test conditions you'll find it in you to do an extra couple anyway.
I'd rest from now on - let things repair, deposit some energy stores. Too late to make much of a difference through training (IMHO), although not too late to make a difference by having a bloody good rest.