I came on F1 visa and graduated in May 2008 (studied 1 semester in 2008) then I started my job as a physical therapist on OPT base for 2 months and I got H1b visa and worked 3 more months in 2008,

1) how do I file taxes?
2) which things do I have keep in mind while I file taxes?

Fairly new about taxes and all that stuff, can anybody guide me in proper way?

Thanks

If you are single, you must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843.

Since you have F1/OPT income you will get deduction based on tax treaty between the U.S. and your country.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)

If you are single, you must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843.

Since you have F1/OPT income you will get deduction based on tax treaty between the U.S. and your country.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)

What if I am not single and I worked in OPT for a few months and the remaining time in H-4 EAD?

Redundant question; already answered.