Sulfuric acid, H2SO4 is polyprotic, not amphoteric. The conjugate base of sulfuric acid, bisulfate (HSO4-), however, is amphoteric. Remember that an amphoteric species can act as either an acid or as a base. H2SO4 can only donate protons, but bisulfate can either can a proton to reform sulfuric acid, thus acting as a base, or it can donate another proton to form sulfate, thus acting as an acid. Keep in mind that the pKa for this second proton is much much higher than the pKa for the first proton, thus reflecting the lower acidity. (-3 for H2SO4 vs. 2 for bisulfate).
As for the second part of your question, consider what is left over when a proton leaves an acid as illustrated by acetic acid, CH3COOH. The conjugate base of acetic acid, the acetate ion has a negative charge which is a base. The more localized the charge is on a base, the stronger the base. The reason why acetic acid is much more acid than a similar alcohol, ethanol, is because it can delocalize the negative charge via resonance. Anything that can help to further withdraw this negative charge will decrease the base strength, thus increasing the acid strength. Good examples of EWG's are strongly electronegative groups like Cl, F, etc. Keep in mind that as conjugate base strength decreases, conjugate acid strength increases via KaxKb=Kw. NO2 is another good example of an electron withdrawing group, whereas NR2 is a strongly electron donating group.
I hope this helps...