What can I do after giving GRE and TOEFL exams?
How should I know which universities should I apply for?


As JeffE mentioned in his comment, you should start by asking your professors, and others at your school what they suggest.  It sounds like you've already been given good guidance to shoot for a range of schools, but you should avoid simply picking from a list and do some serious research into the type of place where you believe you will want to study.


  There are so many good universities in US that it confuses me. What are the parameters that I should be considering while deciding where to apply? 


I would start by looking at the web pages of schools you are considering, and seeing what kinds of research the professors are working on. You may find that this is overwhelming, but I think you'll start to find some places that interest you.  I actually began my search by explicitly thinking about where in the country I didn't want to live for five years, and then went from there (not necessarily the best method, but it gave me some limits to the search).


  I don't want to just randomly apply and get rejected. Is there a way to know it before hand - what things a particular university would be looking for in an applicant?


This is dependent on the departments you apply to, but in general (and this has been answered many times on this site): schools are looking for an indication that you have the potential to do quality research.  The best way to demonstrate this is by having research experience and by having one or more publications under your belt. Letters of recommendation are extremely important, and grades are important, but I would say grades serve as more of a barrier rather than an indicator (i.e., lots of people have the grades, but not all have the other factors that make good candidates).


  Most of the people have suggested me to select 2 ambitious, 3 moderate and 2 safe universities.


This is a good start.  Some might suggest adding more schools (you never know), but this does get expensive and if you are relatively confident you will get into the "safe" schools and believe you will be happy working there, then you're probably on the right track.  Bear in mind that all PhD programs worth the degree do turn away qualified candidates.


  I am referring to this list for all my analysis. But its difficult to know the standards that I should use to decide what an ambitiuos university is. I mean should I consider the top 10 or the top 30 universities as ambitious.


This does depend on your situation, and it's almost impossible to give an answer strictly for your situation.  If you have peer reviewed publications and get outstanding letters of recommendation, you have a good shot at some top universities.  But no one is guaranteed to get into any top-10 program, so don't be surprised if you don't get accepted even as an outstanding candidate.  Again, it comes down to doing some research and talking to people about individual programs.