How to improve credit rating (FICO or similar scores) if i had defaulted a credit card in the past?

I was offered a student card from citibank, around 4 years back. I was careless, the card maxed out. I ran out of money to pay off the accumulated debit and thus, defaulted. It was a small credit limit of 12k rupees ($300 I suppose). I never paid nor applied for any more cards later.
I don't want a bad credit-score, and will need one or two credit cards someday: atleast for online transactions.
So, what do i do? paying the bank now, will it help?

what is credit rating anyways? It is a number given to assess and reflect your creditworthiness. Your credit score will be a number valued from 300 to 850. While numerous factors can be attributed towards this rating, your payment and debt history are more important than others.

To start with, you shall need a copy of your credit report. If everything in the reports is good, then you can be rest assured that you will have a good credit score. Getting your credit reports is easy and pain free. Actually you are permitted one free copy of your credit report, every year. Read more about it at: http://www.credit-card-gallery.com/article/196,Increasing_Your_Credit_Rating