I've been feeling nauseous and lightheaded all day what do i do

This is probably due to low blood sugar. You need to make sure to drink water (in light sips, if you start feeling worse from that, then stop) And eat some food, same thing applies. Use the toilet if you need to, as this can release your bowels of possibly harmful substances.

Sit outside and rest for a moment, this can give you vitamins that may help.

Steer clear of bad smells, those can make you vomit if you are feeling nauseous.

Get plenty of rest, and try not to stare at too many screens.

Heat up some salt on a stove, put it in a rag, and place it on your back-neck.

Consider eating healthier(if you aren't already eating healthy). Sometimes eating unhealthy foods can make you feel sick and lightheaded.

If your condition doesn't get any better or worsens, then call your doctor, or consider getting a check up just to be safe.