Sandpaper is the most common, useful, and widely used forms of smoothing. Essentially it's sand glued to paper. The finer the sand, or the higher the grit, the smoother the end result will be. Larger pieces of sand, or a lower grit, will remove and sand much faster, but will not be smooth.
When using sandpaper, let the paper do the majority of the work. Putting force onto the paper can cause it to rip and dig deep into the surface. Applying a light amount of force allows the paper to do the work and more evenly level the surface. When sanding, move the paper in a circular motion. This will grab and smooth the surface from all angles. Sanding in a line can leave an uneven surface.
Sanding is great for smoothing a surface, but if you are trying to remove paint or only sand certain spots on the object, wet sanding is a better choice. Wet sanding doesn't leave scratch lines from the sandpaper. It sands much smoother, even with higher grit. Wet sanding is great when trying to strip paint, or even a spot where paint has chipped.
Sandpaper comes in various grits. The grit is the general size of the sand on the paper. The lower the grit, the bigger the sand and in turn the faster it removes. This also makes the surface less smooth.
When sanding always go from a low grit to a higher grit. Generally a good grit to start at is 60 or 80. These will quickly remove the larger imperfections. Next move up in grit to around 180. Tis grit will begin to make the surface smooth while also removing imperfections. Depending on what your final intention of the surface is, a grit of 220 or 320 should be enough. These grits will get it smooth for most purposes. However, if you want smoother, grit sizes can go up to around 1500 so there is plenty of ways to make the surface even smoother.
Wet Sanding is great for sanding surfaces that can easily be scratched, or just need a little spot work, or already has paint on it. It also tends to get surfaces even smoother with the same grit. If you used masking fluid to give a chipped paint effect, have uneven paint layers because of masking tape, or sanding surfaces like plastic that can heat up and melt, wet sanding is perfect.
Not all sandpaper can be used for wet sanding. The paper can wad up, fall apart, or the sand can lose its adheason from the paper. Wet sanding paper looks exactly like regular sand paper, and can be used as regular sandpaper, but it won't degrade with water. Hardware stores carry a few grit sizes but auto part stores carry a much wider variety.
Wet sanding is primarily for removing less obvious imperfections and general smoothing. It increases power of sandpaper while also making things smoother. This is why you should start wet sanding at a higher grit. I never wet sand with a grit lower than 220, and generally stay at 320+. To wet sand, fill up a container of water and dip the sandpaper in the water. Get your surface slightly wet as well. Now you just sand like normal. Re-wet the sandpaper as needed to keep everything fairly wet. If you are sanding a pre painted surface, you will see the layers of paint slowly and evenly be removed.