One seat of a bicycle that moves when walking in it can mean only one thing: it is poorly adjusted. The arrangement will depend on the seating style you have and how it moves. A seat that moves from left to right requires a different approach than one that changes its inclination. With the right tool, it's easy to fix.
If you notice that your seat changes its tilt, the nut underneath it is loose. This means that when you ride a bike, only the part of the chair is raised or lowered. Look directly below it. On one side - probably the right - you will see a nut. This controls the horizontal alignment of the seat. A well-placed seat will have a small incline at the tip. Move yours so that the back is just a little lower than the front. Use a pair of tweezers or an adjustable wrench to tighten the nut. You may need to insert a screwdriver into the head of the screw to hold it in place. Do not tighten the nut until the end yet. Get on the bike and make sure the seat is comfortable. If necessary, loosen the nut again and make the necessary adjustments before adjusting it completely.
A seat that moves forward and backward, or up or down, you probably have a loose seatpost. This is the long piece of the seat that slides in the frame. Bicycles have one of two connectors at the intersection of the seatpost and the frame. Newer styles may have a lever on the body that holds the seatpost in place. Some models will have a nut and a screw connection. Once you determine the connection your bicycle has, you will know which tool you need. A lever requires an Allen wrench, while a nut and bolt system requires tweezers or an adjustable wrench plus a screwdriver.
Place the Allen wrench on the hex nut on the opposite side of the lever handle. Turn the handle clockwise to adjust the connection. Pull the lever back until it is straight. This will allow you to move the seat to the appropriate height. When riding the bicycle, the leg in the lower arch of the pedal should stay extended. The leg in the upper arch should be slightly flexed. Push the lever to close it. If it does not fold, the nut is too tight. Hold the nut in place with the Allen key and turn the handle a little counterclockwise. Do it until the lever closes completely. Sit on the bike and check the height. If you need to make any adjustments, open the lever again and move the seat.
Nut and screw
This connection method will require a pair of pliers or an adjustable wrench. Hold the bolt in place with a screwdriver and turn the nut counterclockwise. Move the seat to the appropriate height and adjust the nut. Avoid adjusting it completely until you evaluate the height of the seat. Once you're comfortable with the height, finish adjusting the nut completely. This should keep the seat in place while you ride your bike.