Missing observations

Dear experts,

Hopefully you can help me out as I am already reading online about missing values for two days :). Unfortunately I have quite a lot of missing values for the variables that I use in my regression analysis. Every variable has some missing values, some have more missing values. How can I see how much missing values there are for one variable in Stata 10? And how should I deal with it. My thesis supervisor told me I need the same 'sample' (N) for all descriptive (just a mean of one variable for example) and regression analysis (with sometimes all the variables). How should I decide which variable to drop and which variable to replace by the mean. I know the mean is not a very good method. I also have to look whether there is a difference between the original sample and the missing values. However, I'm a Economics student and not an econometric one so more advanced methods will 'go to far' for me. And off course relevant, how can I do this all in Stata?
Thanks in advance for answering my questions.