Tips are generally expected only in restaurants. A tip between 5%-15% is appropriate for a restaurant bill (some restaurants indicate on menu that the final amount includes a tip, don‘t tip twice) or for a taxi driver.
When it comes to tipping taxi drivers, it is acceptable to simply round up the fare. Sometimes you’ll get a particularly friendly or helpful driver though, in which case you should tip around 10-15% of the fare price.
For a hotel porter tip 1 EURO per bag, for a tour guide a tip of 3 EURO is sufficient.
Tips in restaurants aren’t usually left by guests on the table. Should you wish to tip staff, add around 10% onto the bill or round it up. It’s up to you what sum you tell the waiter you wish to pay when he brings the bill. If you say nothing, you will receive the exact change. If you say the word ‘thank you’ when the waiter is giving you the change, he will understand that the remainder has been left as a tip.