http://en.wikipedia.org/wiki/Basis_set_%28chemistry%29

There should be the most of the things you need to know about choosing the basis set. You should also read chapter about basis sets in one of many computational chemistry or modern quantum chemistry books.

Polarization functions (d,p,..), or * or ** = (d) or (d,p) are contracted primitive gaussian functions added to heavy atoms and hydrogen respectively, so the importance of using them is when you have some polarization effent, like hydrogen electron density polarization in electric field, and it can not be specified with only s functions, so you need to add p functions, and so on. Diffuse function +,++ are added when you have spieces with very important valence electrons, like carbanions, because theese functions are "wider" than regular Gaussian functions, they describe well  electrons that are far from the nuclei.

http://en.wikipedia.org/wiki/Basis_set_%28chemistry%29

There should be the most of the things you need to know about choosing the basis set. You should also read chapter about basis sets in one of many computational chemistry or modern quantum chemistry books.

Polarization functions (d,p,..), or * or ** = (d) or (d,p) are contracted primitive gaussian functions added to heavy atoms and hydrogen respectively, so the importance of using them is when you have some polarization effent, like hydrogen electron density polarization in electric field, and it can not be specified with only s functions, so you need to add p functions, and so on. Diffuse function +,++ are added when you have spieces with very important valence electrons, like carbanions, because theese functions are "wider" than regular Gaussian functions, they describe well  electrons that are far from the nuclei.

You can find a good description about basis sets in the books written by errol lewars and another book written by Frank jensen. usually for 1st and 2nd row elements most widely used basis set is 6-31g(d), choice of basis set depends upon the property you wish to calculate. Inclusion of higher polarize functions like d,f,g will consider the correlation effects of atoms and molecules. Diffusion functions are generally used when we choose to study systems like anions.

Choose a lower basis set perform optimizations and calculate Single point energy calculations with higher order basis set, since you will not notice much difference between optimized orientations even if you employ a very large basis set and will be computationally expensive. This is the best approach to perform calculations

Apologies for being so bold, but what is the point of doing such calculations in your undergraduate study if you obviously don't know the basics? Even if you will learn here what diffuse functions are, will you be able to judge from the output whether the results are technically correct and hopefully meaningful in answering some question?  Don't get me wrong, I just want to encourage you to learn exactly  what you are doing, otherwise running such calculations is like a computer game. 

i suggest you to use b3lyp 6311++dp  for second row elements if this doesnt work  then try  these combinations with b3lyp

6311+d,p 

6311+d

but if u r having a transition metal try using b3lyp with lanl2dz or b3lyp with lanl2mb combination. i hope it helps.

try reading cramer for computational chemistry its a good book and i hope this will prove helpful to u .