steps to upgrading my mk5

okay im new to modifying engines of cars and besides making sure your car is up to date and clean(meaning oil changes and clean hoses) what other steps should i make before starting to upgrade? 2006 mk5 jetta tdi 115k miles

On that engine? Inspect the condition of camshaft and lifters very carefully. And make sure you don't have a big exhaust leak at the change-over valve for the EGR cooler. The dual-mass-flywheel (part of the clutch) is another potential failure point.