In disk utility chose the drive and then click erase.
Under security options you can select the level of secure erasure. Be aware that if you choose Most Secure it will take hours. I usually choose the second level as this overwrite with random data then with all zeros.
When you have finished you will need to select the file format for the drive. If you have a need to access the data from a Windows machine at a later date then ExFat is the best choice.