make sure you have enough weed to roll an entire joint - you can't roll small joints, you roll a joint, or a big joint - its as simple as that
go to youtube and search for how to roll a joint videos and shit, just watch how they move their fingers, etc.
just keep practice rolling - you'll waste some papers, but practice using rolling tobacco or tobacco out of cigarettes just to get the hang out it without using your precious greens
if you don't have long nails, sometimes i find it hard to tuck the paper over the substance in the middle being rolled, so i'll use a credit card to tuck it and then finish rolling by hand
but just practice what you're doing, rolling is a process that you don't just "have" - you need to learn it, and it almost becomes an art form if you value your smoking