For being an entrepreneur, you need an idea and persistence to work on an idea. Believe me, degree doesn't matter a lot but yes the network and experience help. So if that is your aim, probably a good college is a better idea
If you like marketing --> go for marketing, if finance --> choose finance.
SPeak to people around, people who are doing an MBA or people who have done an MBA, try to get a sense of which area suits you better both in terms of your strengths and your liking. Go for it. It matters in the end that you are doing what you like, that's when you will be able to enjoy that!
"is it sales or marketing manager directly or a small post is given after completing an mba for say from a reputed institute such as sp jain" -- this varies from company to company. Some select you for manager position, some for trainee but what is important is how much you like what you are doing and then it doesn't take long to move up the ladder.