I can't see/use my SSD hard when i open my computer (windows 8), also i tried to use disk management and i see the following:

All of them is healthy !!......but i can see the only drive with letter (C) on my computer......i want to know how to add letter to SSD hard and use it in my computer......also how to change the windows to be on SSD instead of the HDD to get the advantage of the SSD !!

You first need to initialize the disk and bring it Online.  In Disk Management, right-click the disk you want to initialize, and then click Initialize Disk.  Your disk should be usable once you format it and chose a disk type.

As for how to get the most out of your new ssd I would attach a vhd, image your operating system onto the vhd then mount the vhd onto your ssd.  See

http://technet.microsoft.com/en-us/library/dd851645.aspx

on how to create a vhd and mount it onto a drive. 

You could also follow this tutorial to avoid the vhd issue.

http://www.sevenforums.com/tutorials/149969-ssd-install-transfer-operating-system.html

You can use backup and recovery in the control panel to make a system image of your OS.  Make sure you have enough space on your drive plus a little extra for any future applications you think you might install.  Also, make sure you know what partition table type you need.  Windows 8 I believe is all GPT not MBR.  Make sure you know before you go trying to install an image of a GPT table onto