How to...

There are three main approaches to valuing a business.
1. Market approach
You look at the value of similar companies as compared to some financial parameter (usually EBIT, EBITDA, net profit, sales, book value of equity etc.) and then apply this multiple to the financial parameter of the company you're valuing to estimate its value. A simple example:
Company A, which is very similar to company B that I'm trying to value, is worth 100 and has EBITDA of 10. The Value/EBITDA multiple is 100/10 = 10. Company B has EBITDA equal to 12. Using the multiple of company A we can estimate the value of company B to be equal to 120 (12 x 10).
2. Income approach
You prepare financial projections of the company for the future. You calculate free cash flowand estimate its present value. This is theoretically and practically the best and most appropriate approach to use, since it yields the most "correct" result (and we'll talk about getting to the "correct" result as well).
3. Asset based approach
The most static approach, useful only for holding companies or companies in liquidation. You estimate the value of assets (as if sold separately) and deduct the value of any obligations.

I'll go through these approaches into greater (but not too great) detail and also touch upon other topics (liquidity, minority vs. majority ownership stakes, valuation of start ups,...).

Feel free to suggest other, even more specific topics of interest that I can elaborate on with respect to business valuation and/or ask questions that I will try to answer.