Need advice

Start with VAT & Service since the amount of reading you have to do in VAT & ST is pretty less, but the weightage is 40% (I can say the entire syllabus of Service Tax and VAT can be completed in just one day). Then take on Excise (the syllabus is a little vast; but still requires great attention since it also has 40% weightage) and lastly Customs (don't spend much time on this; just give a rough reading). 

Good luck!!!

thnks shrikant, could u suggest me which book to refer..

Not sure. But I'm using Yogendra Bangar's book and I find it okay; I mean not bad.