Hi Fededito,
I really cannot believe I am providing an answer on this forum
Anyhow to respond to your question, there is no minimum balance needed at IB to have Paper Trading access. However, if you deposit less than $2K, they will charge you $20/mo fee which can be offset by commissions. If you deposit between $2K-$100K, they will charge you $10/mo fee which can be offset by commissions (ie. if you generate $2 in commissions, then their fee is $8). If you deposit $100K+, then there are no monthly fees.
Now to begin trading, you will need $10K or your trades will not execute but you can still enter them. Unless you are younger than 25 years of age, then there is only a $3K requirement. More details are on this URL:
http://www.interactivebrokers.com/en/index.php?f=4969&ns=T
Hope this helps. And thank you for asking this question as I learned as well because I had to do research to find this answer.
Thanks,
Praveen