DU admission for BA : The problem is that I'm a Science student who wants to join an Arts college after 12th.

So, what's the problem? 

There is no law that says that a Science student cannot join an Arts course for undergraduation!! By all means, go ahead! But just ensure that you have a certain amount of knowledge/ skill set to pursue it further and not get stuck mid-way!

So, choose your specialisation carefully. Honours degrees in English, History, Philosophy, Sociology, Journalism & Mass Communication, Political Science etc. should be easy for you to catch with, given that you would not have studied them in Class 11 & 12.

But courses like Geography (H) etc., which are a little technical in nature, might be a bit of a struggle for you. Explore!