For example, I have the Schengen visa issued by the embassy of Germany in Turkey. I've gotten the exit stamps on my Turkish passport and boarded the plane at the Ataturk International Airport in Istanbul.
Also, I have a Sharp CD-PC1881V video CD mini system inside my luggage. Should I declare the Sharp mini system/small home theater when I arrive at Frankfurt International Airport in Germany?
I also have a laptop computer with full battery in the same luggage.
The general rule for dealing with customs anywhere is: If in doubt, declare it. In EU airports you do that by following the red "goods to declare" lane after the baggage claim. Sometimes the red lane will be unstaffed and you need to push a button or use a provided telephone to summon a customs officer.
The customs people will then figure out whether they're interested in your items. If it turns out that you could legally have brought them in without declaring them, you will be told so, and then the whole exercise won't cost you anything other that your time.
Also, if in doubt, travel with the best documentation of the value of your items you can lay your hand on -- in particular receipts or invoices showing how much you paid for them and when/where you bought them will be useful.
For example, I have the Schengen visa issued by the embassy of Germany in Turkey. I've gotten the exit stamps on my Turkish passport and boarded the plane at the Ataturk International Airport in Istanbul.
First note that in the EU customs and immigration are seperate.
If you are transfering to another non-Schengen flight you will not normally go through immigration or customs.
If you are transferring to a Schengen-internal flight you will normally go through immigration but not baggage claim or customs. You will then go through customs at your final destination airport.
If you are leaving the airport you will normally go through immigration, then baggage claim, and finally customs.
Immigration is about you, they will check your visa, ask you some questions about your visit and assuming they determine you are a legitimate visitor they will stamp you in.
Customs is about your stuff, what it is, what it's worth and under what circumstances it is brought into the country. You will normally have a choice between a green and a red lane. If you have nothing to declare then you take the green lane and can normally just walk out. If you have goods to declare or have customs enquiries then you take the red lane and speak to a customs officer. If you catch a connecting flight to a place that gets few passengers from outside the EU then you may get a red phone instead of the lanes system.
If you are visiting you don't normally need to declare reasonable personal effects that you are importing temporarily. ( http://www.revenue.ie/en/customs/leaflets/cdpn46.html is for Ireland but I believe it's much the same across the EU. )
If you are bringing the goods in permanently for personal use or as a gift then value-based limits apply (currently €430), if your goods are under the limit (and aren't in a category subject to special restrictions) you don't need to declare them, if they are over the limit you have to declare them and pay duty/VAT.
If you are bringing the goods in permanently and commercially then you have to pay tax however small their value.
If you are bringing stuff in temporarily that are not reasonable personal effects then you are likely to have to do formal temporary import paperwork.
Also, I have a Sharp CD-PC1881V video CD mini system inside my luggage
That is a pretty unusual thing for a traveller to be bringing with them. It doesn't look especially portable.
You should certainly try to find some evidence of its value (or lack thereof), especially if you intend to import it permanently.
