I am new to Blender. I think i accidently click something and my model become like this. The character's front view appear at side view port and character's side view appear at front view port. What should i do in order to change everything back to normal? i mean turn character's front view back to front view port.
Rotate your object to make it fit the viewport - select it, press R to start rotating, right after that press Z to limit rotation to Z axis, next type in 90 on your keyboard to make it rotate 90 degree along Z axis.
Try it out, it might be I am giving you wrong degrees out of my head, just experiment, it should be fairly easy to figure out.
After that you might consider applying your rotation to the object (so it thinks that's its default rotation he is in right now) by pressing ctrl+A and selecting rotation.
You can also press N hotkey when having mouse cursor centered on 3d view - this way you will be able to see rotations, translations and scale under Transforms panel, and change them however you like.