There are a few different ways that you can change your velocity on Roblox. The first way is to use the built in function setVelocity(). This function takes two arguments, the first being the x component of your velocity and the second being the y component. You can use this function to set your velocity to anything you want, including zero.
The second way to change your velocity is by using forces. Forces are what cause objects to accelerate, and you can apply them yourself using the ApplyForce() function. This function takes three arguments: The first is the force vector (the direction and magnitude of the force), the second is the point at which to apply that force, and third is an optional parameter for how long that force should be applied. If you omit this last parameter, then the force will be applied for one frame only.
You can also use impulses to change your velocity. Impulses are like forces, but they're applied instantaneously instead of over time. They're often used for things like collisions, because they simulate what would happen if two objects hit each other with a lot of force all at once. To apply an impulse, use the ApplyImpulse() function. It works just like ApplyForce(), but with an additional argument for how much mass should be affected by that impulse (the default value is 1).