Using fdisk is fairly simple in this case: start it via "fdisk /dev/devicenameOfDisk". "m" lists all available commands, "p" lists the current partition table, "v" does some checks of the current setup, "n" creates a new partition, "q" will quit and "w" will write the current config to disk.
So what would seem like a good sequence:
1. "p" to see the current partition layout (you should only see a single partition there)
2. "v" should tell you about plenty of unallocated sectors
3. "d" would delete that single partition
4. "n" (plus follow-up values, all default I guess) should create a new partition spanning (mostly) the whole disk
5. "p" should show a single partition of the same type and starting point as from step 2, and a much higher end point/size
6. "v" should report close to no unallocated sectors
7a. if unsure at this point, "q" to quit.
7b. If you feel safe, use "w" to write to disk. fdisk will quit and tell you that the kernel is reloading the partition table.
8. Re-run fdisk to use "p" verify everything is as you assume. If you received a message that the partition table could not be reloaded, you'll have to reboot before the next step.
9. "resize2fs /dev/yourPartition"
10. mount & enjoy
Standard disclaimer applies: Use this at your own risk and always apply logic to my instructions (*never* trust me )