Use whatever grip is strongest for you - palms facing, palms away, palms parallel. You can mix them up. If you can't do many (or any) work on negatives (jump up to top position; lower slowly); use a Gravitron if you have access, or even work on pulldowns until you are stronger. Beware, however, that pulldowns are much easier than pullups, even when pulling bodyweight. Best is to go for the negatives, or multiple sets of one or two.