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There's no set rank for joining the clan. Some clans only accept higher ranked players, but there are clans that also take in lower ranked players. Although, I wouldn't start looking for clans before reaching at least Warrant Officer ranks.

You can check whether your Z key is working by using this online keyboard tester. Also check whether you are using QWERTZ or QWERTY keyboard. Your Z and Y keys might be inverted.
Additionally, if the problem for some reason still persists, you can now freely customize your tank controls in game settings.

Hello. It seems like you have problem with loading the flash plugin. Try these solutions; https://support.google.com/chrome/answer/6258784?hl=tr (I presume you're Turkish judging by the error message)

I am not sure why it is there, but with 99,9% probability crystal boxes won't return to the game. They were removed because they distracted tankers from the main battle objectives.

Load the game again and press Ctrl+0.

Hidden chambers, invisible getaways, labyrinths and such are available only for short period of time, usually when traditional Tanki Online contest 'The GAME' comes up. Otherwise, there are few hardly accessible places on few maps that can be considered more or less as 'secret places', but they have been mostly removed. However, you can check the current map views here and assess whether they fulfill your definition of 'hidden place'.

Try clearing your browser cache and flash cache and running only a minimum of other apps at the same time.

It costs 410 610 crystals at 40% sale. You can find all the information about the product kits on the official Wiki page; http://en.tankiwiki.com/Product_kits

Only players below Warrant Officer 3 can buy M0 Striker. Since you are Second Lieutenant, you have access to M1 Striker (unlocks at WO3). You can now buy only the highest modification available at your rank due to this update: http://en.tankiforum.com/index.php?showtopic=285725