Tomorrow I'm going to ATS for the first time. I feel prepared. However I was wondering about dress code. Should I go there in shirt and tie or will casual clothes do? Also any other recomandatios and suggestions are very welcome and appreciated.
ATS does not care. All they care for is that you pay.
Avoid vibram or other toe shoes, the gym floors are hard and shoes lacking a proper sole will result in ankle pain.
You have gotten good advice above about the gym session. As for testing, wear something that you like and are comfortable in and doesn't make you look like a douche.
Bottom line. If you are comfortable in a tie, wear a tie. Or not. No one is going to care. Just don't look like a douche.
When you get there, you would be surprised by the amount of people who actually think sweatpants are appropriate attire to wear in public. It really doesn't matter if recruiters are there or not at ATS sessions. (And, yes, they are not.) It's not an all-night WalMart at 3AM.