There are many different ways to go underwater, but the best way is at high pressure and low temperature so that your body’s air bubbles will not pop. You can either buy or rent a set of diving gear and take an introductory dive class with a certified instructor, which should get you ready for scuba diving no problem (depending on swim levels).
You can hold your breath, and dive underwater as quickly as you can, but if you do this then it may only last for a few seconds because the air in your lungs will start to get uncomfortable. You can use equipment to stay down longer. Equipment like scuba tanks will allow you to breathe at the same time that they supply enough compressed air so that when exhaled it creates buoyancy in addition to breathing air mixture in between dives (normally there is an automated regulator with built-in decompression valve.) You may also explore using a snorkel – keeping your mouth and nose above water while swimming downwards with fins on feet or hands – until you reach desired.