You can't do it safely while the partition is mounted, meaning you need to boot some other partition and do it from there.
gparted is a nice, easy GUI for this purpose. In our deleted comment exchange you mentioned it would not start because of "can't access display" -- this implies you aren't in X; since it is a GUI it won't work without that.
Of course, if you don't have another partition to use, you'll need a live CD or something -- I think they usually come with gparted. Your best bet is probably the actual gparted live CD, which looks to have a reasonably recent latest stable version (and will fit on a CD, which is nice since the "live CD" is rapidly becoming the "live DVD").
I've never had gparted cause a problem but of course do back your important tish up first.
All that said, a quote from from man resize2fs may be of interest:
The resize2fs program will resize ext2, ext3, or ext4 file systems. It can be used to enlarge or shrink an unmounted file system located on device. If the file system is mounted, it can be used to expand the size of the mounted file system, assuming the kernel and the file system supports on-line resizing. (Modern Linux 2.6 kernels will support on-line resize for file systems mounted using ext3 and ext4; ext3 file systems will require the use of file systems with the resize_inode feature enabled.)
Keep in mind though, this is about filesystem expansion which is a distinct operation from partition expansion (which must be done before filesystem expansion). So, if you can get whatever tool to expand the partition while it is mounted, being sure to keep the exact same starting block, you could expand the rootfs in place.