Modeling
You model the 3D thing and then slice it.
Most slicers take STL files as input. These files are nothing but a bunch of triangles that are the outside of the object. There are nothing but straight lines in an STL file. A circle is made up of a bunch of triangles. When the software puts out a circle it can use a few or a lot of triangles. There can be problems with these files, look up manifold.
Some slicers take STEP files. These files tell how to create the 3D object and they may or may not be made of triangles. STEP allows for curves, but does not require them.
There is no easy thing. Creating a 3D object is not easy. The first thing you have to decide is what type of thing do you want to model. Is it an organic thing, like a teddy bear, or a mechanical thing, like a box. The “best” software for these is different. If your answer is both, then you will probably have to use two different software packages. If a thing is good at organics shapes it is not good at exact shapes.
If you want an organic thing it simple. Blender is for you. It’s free and there are a lot of tutorials in words and on YT. I do not teach Blender.
Mechanical things are a little different. You can spend real money for a parametric CAD package. Most of them also have versions for hobbyists that are free with some restrictions that make it unappealing to industrial customers who pay the big bucks.
TinkerCAD is a little different. It is not quite a parametric CAD package and it has some limitations on what kinds of objects it can create. It is free. It is easy to use. If you do not run into the limitations it’s OK. It does have limitation in what it can produce and when you run into them there is no way around. I tried tinker and ran into limitations and now use OnShape.
The real big boy CAD programs are OnShape, Fusion, and some others. I do not teach any of these nor will I get in the middle of a religious argument over which is best. In general you draw a 2D thing and then extrude it into a 3D thing. You can add new 2D drawings to a face and extrude that. Add fillets and chamfers. If you make a change back in the original 2D drawing the software will recreate the 3D thing. For example the 2D drawing could be a simple box and you can change the length and, as if by magic, the software will create the 3D shape with the new size.
No matter what program you built the 3D object in you can export it as an STL, and possibly a STEP. Bring that file into the slicer.