The most Important factor in a certain algorithm

[Charleston]

I have read many posts I searched on these forums about algorithms, and had a look at some materials about how to analyze some simple algorithms. I however still do not understand how I can determine the most important factor in a certain algorithm. Especially in case there are many a loop and many different computations in those loops. How will I know Which one I have to choose among them then ?

Anyone please help me ?

Thanks a lot,