So first, on which sites are you trading? Im currently on Mt. Gox but Ive read on one of the first page that its better to have 2 accounts, one for short and one for long
How can you do that? I mean you have to buy when its going up and sell when its going down.
At the time of writing, Bitcoinica was an alternative exchange which allowed shorting. I have no recommendations as to any of the exchanges which will allow it these days - in other words, I haven't done the due diligence necessary to trust any except Mt. Gox at the moment, and they only allow buying of BTC.
Second question its more a market/trading thing
If for example I want to sell my BTC, imagine now at the current buying price (atm 124$ / 3 BTC available), if I sell 2 BTC, theyll be sell immediately. But now if I want to sell 5 BTC Ill sell 3 immediately and have to wait for someone to complete my offer? Thats it? But Ill still have 3BTC available for trading.
If for example I want to sell my BTC, imagine now at the current buying price (atm 124$ / 3 BTC available), if I sell 2 BTC, theyll be sell immediately. But now if I want to sell 5 BTC Ill sell 3 immediately and have to wait for someone to complete my offer? Thats it? But Ill still have 3BTC available for trading.
I'm not exactly sure what you're asking here. I think what you're trying to figure out is the difference between a market and limit order. Read this and let me know if this is what you're looking for:
http://www.scotiabank.com/itrade/en/0,,3969,00.html
http://www.investopedia.com/ask/answers/04/022704.asp
I also wanted to thank you for this journal that is very helpful for me and it allows me to start trading with a strategy. Ill of course study this strategy myself and change some settings. Could you tell us more about the turtle strategy? Or give us some links? 
Glad you enjoyed it! Here's some information about the strategy:
An overview: http://www.investopedia.com/articles/trading/08/turtle-trading.asp
In-depth discussion: http://bigpicture.typepad.com/comments/files/turtlerules.pdf
C'est pas grave pour l'anglais. C'est pas mal! Best of luck with your trading!