i just started to learn about Computer Organization and circuits. Is there a good free software where i can build my own computer and Design my own circuit boards?

Have a look here - Open Circuit

Are you familiar with Linux? There are some interesting free packages available for Ubuntu.

Has your tutor recommended anything? If someone is teaching you, then it would be a great help to you if you used the same package. For example, if your tutor wanted you to place a particular component in sequence with another, you may know how to do it in theory, but a different software package may complicate your doing this - and your tutor would not be able to advise you because the layout of the features (menus, options, etc) are different to his/hers.