You can find the answer right here on the askubuntu site via the answer to this question: How can I increase a partition's size.
I don't mean to be rude, but I do feel like I should point out that search is your friend in answering questions like this. Both Google and the search box up there in the right hand corner of askubuntu would have delivered you an answer in just a few seconds--probably less than the time it took to enter the question.
Anyway, just follow the steps to boot with a LiveCD and run gparted, and you should be all set. PLEASE back up your data first though!