GetBytes() returns negative number

GetBytes() returns negative number (-61, () ) at the char 'ä'.

Well getBytes() is going to use the platform default encoding, unless you specify an encoding, which you should. I would recommend UTF-8 normally. For example, in Java 7:

byte[] data = text.getBytes(StandardCharsets.UTF_8);

byte in Java is unfortunately signed - but you can think of it as being just 8 bits. If you want to see the effective unsigned, just use:

int unsigned = someByte & 0xff;

How to get the normal ascii value?

That character doesn't exist in ASCII. All ASCII characters are in the range U+0000 to U+007F.