Ensure having your important datas backed up before wiping your phone, such as important contacts, picture google accounts e.t.c
Press and hold the correct keys to boot the device into recovery mode. This will vary from device to device. Here are some examples:
Nexus 7: Volume Up + Volume Down + Power
Samsung Galaxy S3: Volume Up + Home + Power
Motorola Droid X: Home + Power
Devices With Camera Buttons: Volume Up + Camera
Similar devices will likely use similar key combinations. For example, the Nexus 4 also uses Volume Up + Volume Down + Power.