Whether you like to cycle to compete or just for the pure joy of it, a proper bike fit can make all the difference in how you feel when you get off the bike! You do not have to do a bike fit in person in a shop though that is one option. You can opt for a virtual bike fit either way you will be more comfortable, faster, and less likely to have any injuries or accidents. While it is a bit more complicated than this you can say there are three main categories to focus on, handlebar height, seat height and then seat positioning.
The general suggestion is that handlebar height should be 2.5cm to 7.5cm lower than the saddle height. This can vary though depending on back flexibility. You want to have a slight bend at your elbows and not have a lot of pressure on your palms. Handlebar height is probably the more subjective or personal choice in an in-person or online bike fit. It depends on what you can maintain in terms of back bend, that means you remain comfortable during the ride still. A lot of riders find that as they train their flexibility improves and then they can reduce the height of the handlebars by a small amount. If you have aero bars you should have your forearms level from a side-on view and your ears should be over the elbows, you should have an angle at the elbow of 90 to 110 degrees and experiment with elbow width until you find the most comfortable at the best speeds.
With your seat height, you should start at the highest setting that still gives a smooth ride, so your hips are not rocking as you reach the bottom of the pedal stroke. Your toes should not be pointing hard to get that full turn of the pedal. Sit on the seat and stretch out your leg as far down as you can locking in the knee. Keep the foot parallel. If your toes have to point down and your leg is fully stretched then your seat is too high. If your heel has to go down to get the leg to extend then the seat is too far down. When a seat is too high it will impact the power you can ride with. But a seat that is too low can put force on your knees and you could experience pain and injury to the joints. That is why this or a virtual bike fit is so important.
The seat fore-aft position is next. When the crank is at 90 degrees you should have a straight drop from the front of your knee to the middle of the pedal axle. To change the fore-aft position you need to look at the angle of your seat tube. Usually, they are between 72 to 78 degrees with more experienced triathlons using a higher number. It means your hamstrings are more involved as you pedal and this means it is easier to achieve the desired higher cadence. Having a seat tube angle that is too steep can lower your speeds though. Try an online bike fit for more information and personal attention.