To type in the secret codes, you just have to open the default dialer and type in these codes.
Here's the list of 20 Secret Codes
Phone Information, Usage and Battery – *#*#4636#*#*
IMEI Number – *#06#
Enter Service Menu On Newer Phones – *#0*#
Detailed Camera Information – *#*#34971539#*#*
Backup All Media Files – *#*#273282*255*663282*#*#*
Wireless LAN Test – *#*#232339#*#*
Enable Test Mode for Service – *#*#197328640#*#*
Back-light Test – *#*#0842#*#*
Test the Touchscreen – *#*#2664#*#*
Vibration Test – *#*#0842#*#*
FTA Software Version – *#*#1111#*#*
Complete Software and Hardware Info – *#12580*369#
Diagnostic Configuration – *#9090#
USB Logging Control – *#872564#
System Dump Mode – *#9900#
HSDPA/HSUPA Control Menu – *#301279#
View Phone Lock Status – *#7465625#
Reset the Data Partition to Factory State – *#*#7780#*#*
Format Your Device To Factory State(will delete everything on your phone) – *2767*3855#
Hidden Service Menu For Motorola Droid – ##7764726
If you know any more secret codes, then comment it below. Share with everyone and let them play with it.