Hello, I’m a24 years old guy and I’m looking for advice from more experience men. What are the common mistakes to avoid? Any advice for talking to the girls and how to treat them, is there any dress tag or recommendation? or maybe which providers would you recommend me in Germany and Netherlands. Any advice and comment is welcome. Thank you in advance.

The FKK clubs in Germany have their own forum here on TER.  You should check the forum out.  These are sex clubs where I guess you pay to enter then select from the talent inside for extra money.  
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For any escort, just be a bit light hearted in conversation.  You don't need pickup lines, etc.  The  only thing I would watch for (besides upselling) is that they jabber too long rather than getting down to biz.  I would say if you are talking longer than 10 minutes and nothing is happening, it is time to make a move.  I'm sure they'd like to burn up the time just gabbing, so you have to guard against that.  The moves are simple, for instance, slide closer, begin to touch them such as stroking their hair or shoulders, etc.   This is the signal that you want to get down to biz.  Most will kick into gear at this point.  But if you have to proceed, you can slowly move your hands to more fertile areas, so to speak.  Light kissing, etc.
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Considering you might be with people who speak a different language, I don't think you need to worry too much about what to say.

The reason you (and many others) are in this world is because you're uncomfortable talking to women, especially about sex. That's fine, but as long as you're courteous there's not much that can go wrong. And once you've done it a few times, the confidence will come. There's really not much to worry about.
In the end, they are just human beings. They don't hang upside down in their closet like a bat until you summon them.

On the website (link below) you can book tours thru them. It will cost you a bit more, but you will have English speaking guide - not to mention some American company.  I am assuming you are in this country for a while.  

as long as he get it  
but the payment not until the deal is consummated.

Much like strip clubs here.. We get our 5 min of dance and then we pay the strippers. No difference. Only there it is sex.. Here it's a dance.

Realize this board is dominated guys in the USA. I would research Germany and the Netherlands somewhere else. My understanding is that getting at escort in Germany is like getting a high-priced haircut in the USA. No legal bullshit.
Have fun and just be respectful without taking yourself too seriously.