There are two types of taxes you need to worry about with Bitcoin.

The first is regular income tax. If you earn bitcoins by mining or doing any other kind of work, you must declare the market value of your bitcoins as earned income whether you sell them or not. In some jurisdictions (I'm not sure about Greece), this also applies to gifts and gambling winnings. You do not have to pay income tax if you simply bought your coins on an exchange (but see below). You may also be able to claim a deduction on any expenses relating to your bitcoin earnings.

The second is capital gains tax. If you sell or otherwise trade your bitcoins (regardless of how you acquired them), you have to declare the difference between the market value when you acquired your bitcoins and the market value when you disposed of your bitcoins (eg, if you bought BTC100 for €2000 and then sold BTC2 for €200, you would have to declare a capital gain of €160, since you bought your coins for €20 each and sold for €100 each, so you made €80 profit per coin). If the market value when you disposed of your coins is lower than when you acquired your coins, you have a capital loss which you may (again, I don't know what the law is in Greece) be able to claim a deduction on.

I'd advise you to consult an accountant (instead of random people on the Internet) for further clarification of your tax situation (especially regarding capital gains tax, which can get quite complicated). As you point out, if you deposit large sums of money to your bank account, the authorities will find out about it, so be sure to declare it correctly on your tax return.