[SOLVED] Integral convergence and divergence:How to choose comparison function?

I'm having difficulty understanding direct comparison test and limit comparison test for

determining integral convergence and divergence(Crying). For direct comparison test how do you

choose the second function?

Can I arbitrarily choose any function for comparison with the given function Or is their

any criteria for choosing the function? Also for limit comparison test how do you

figure out the second function? Any help will be appreciated.