What should I do if diagnosed with tachycardia?

Totally depends. If your heart is normal on routine testing, and you have "sinus tachycardia" (i.e. Normal rhythm just faster than most), often nothing need be done. But first you need thorough medical exam to make sure no other disease is causing this. If the rhythm is abnormal, medications are often related to control the heart rate and/or symptoms. Catheter ablation can be curative sometimes.
Tachycardia. Ask your doctor to explain it to you and what you need to do.