I can't enter recovery mode
Please use  hard reset key method to go into boot loader option . Some devices have two key method some has three . For sony Xperia device it was camera + volume down + power . Please find what is for your device model. 

For Xperia C device:
Xperia C device doesn't allow user to go into bootloader using hardware key for this you have to unlock bootloader first .
But you can use flash mode to flash your rom which are as follows : 
1) open adb shell .
2) connect your​ device by pressing volume down key to  laptop​ using usb cable.
3) you can now run scripts on adb shell to install custom roms etc.

Update :
Now when you unlocked your boot loader the official website will guide you to download few drivers and software. I will explain you that with details :


Now download android sdk which link are mentioned on website . Link

It is not mandatory to download android studio you can also download SDK without android studio IDE.

SDK : Official Link / filehippo Link
 Now run sdk manager 
 Here you will find all the tools mentioned on website to download . Download and install them :

 Now go to the downloaded folder of Google USB driver:
Replace the original android_winusb.inf-file with the downloaded file in the usb_driver. ( which you have downloaded in step a.2 )

 Make sure you have installed ADB driver for your xperia C . 
Link

Now proceed further . I hope you will not get any error after this .