I am not able to use some special symbols like for theta, alpha and all similar symbols. Also, how to get on with the exponent and fraction?
First, check whether the site you're posting on actually supports MathJax: Which Stack Exchange sites use MathJax?
There's a tutorial/reference guide on Mathematics Meta; there you will find how to produce the right symbols and formatting: MathJax basic tutorial and quick reference.
If MathJax is not enabled on the site, you can simulate a fraction of it with HTML entities and Unicode. This very site does not have MathJax enabled, but writing
α produces α, and
2<sup>3</sup> produces 23. Fractions don't work, but small fractions like 1/2 can be written in plain text as well.