Just have to keep at it with the pullups/chins till you can manage to do more.
I went from being unable to do a single chin to getting 5x5 comfortably in a pretty short space of time by using a technique called "greasing the groove". You will need a place to do pullups/chins at home. If as you say you can't do 1, then start by doing a couple of negatives (ie jump up and slowly lower yourself), but do it 5-6 times a day. Each set shouldn't be taxing, like i said you are only doing a couple of reps at a time. Within a couple of weeks you should be able to do 1-2 full reps, so now start doing 1-2 reps 5-6 times a day (again the set should not be too taxing). It won't be long before you can get a decent amount out in one go.