Present continuous or present simple?

Present continuous should be used when something is happening now for a limited amount of time.

For example:

Can I call you back later? I'm eating dinner.
Take an umbrella with you, it's raining.

Present simple is used for a fact (something that is always true) and for a routine.

For example: 

I have one sister.
I get up at 06:45 on weekdays.

The brewery in Vienna smells a bit like peanut butter.

Water boils at 100°C.

Present simple is used to describe results.

This shows that...
It indicates...
The sample consists of...

Origin of the confusion:
Present continuous doesn't really exist in German (ich bin dabei, etwas zu machen).  
Native German speakers who speak little English generally don't use present continuous. For example, if you asked "what are you doing" they might reply "I cook" (the correct answer is "I am cooking"). Or if you are talking in the cinema they might say "be quiet, the film starts" (the correct phrase is "be quiet, the film is starting").
Probably because they are afraid of making this mistake, native German speakers who speak good English generally use present continuous too much. They might say "your sister is looking a lot like you" (the correct phrase is "your sister looks a lot like you") or "I am sleeping a lot at the weekend" (the correct phrase is "I sleep a lot at the weekend").