I have installed Ubuntu 16.04 LTS alongside windows 8.1 on my dell inspiron 3551 laptop with USB. The installation process is successfully done. And then it ask to restart the computer. when i restarted the laptop then it doesn't showing the grub it directly opening windows operating system.
You should check, what works in BIOS-settings of your machine:
Reboot your machine and press key 'F2' (or key 'F12'?). Then you are in settings of BIOS. Check there the booting-order in menu of "boot". There you can put your hard-disk at first place in first line.
Then exit and save settings. Reboot and good luck.
When Windows is still booting first, you should edit the grub entries. This is explained here and duplicate of this link here: