Bust: Relax your arms and measure around the fullest part of your chest.
Underbust: Measure directly underneath the bust.
Waist: Measure around the smallest part of your waist, which is usually around the belly button area.
Hips: Standing naturally, measure around the fullest part of your hips.
Factor in how you would like your clothes to fit. For loose fit or clothes that might not stretch for example, you may want to go larger than usual.
You can also measure similar style clothing you might have at home where you like the fit.