To boot into Recovery mode, start up your Mac and hold.
If you’ve already installed El Capitan and you want to format you Mac’s internal drive, you can boot into Recovery Mode to format the drive. In the top menu bar, go to View, and choose Show All. Please See Answer 20985: How to access Disk Utility macOS 2.
Instructions below are for 10.13.x (High Sierra) 1.
Verify Volume Format is set to MS-DOS file system or exFAT, Scheme is set to “Master Boot Record” (Do NOT select GUID Partition Map) then click Erase.