As said above: there is not an easy (1 line or so) way to set the global precision somehow. Mathematica has two types of floating point numbers: one is machine precision and is arbitrary precision. Type machine precision numbers (1.35, 4.2318 et cetera) and the calculation will continue with machine precision numbers. Type arbitrary precision numbers (1040 or 4.15
60) and the calculation will use arbitrary precision all the way.
You could, though, ask Mathematica to pre-process everything that you type (through \
$Pre or \$PreRead) in by first applying a function that sets the digits of each number to a specified value.
This is also easy to find on something called 'Google':
check this out for example