Hi Eve,
I think it would be helpful if you could post here a picture of an example sign that you would like to make, then people here will be able to give you better advice.
If you are just starting with the Shapeoko, you should start with Carbide Create software (you probably already downloaded it as it is the natural companion software for the Shapeoko?), it probably has all the basic capabilities you need to get started and experiment with cutting ABS/HDPE sheets.
Most of the time you will want to carve features following a given design/shape/logo, and then cut the outline of the part. For this, the best to cut plastics are “O-flute” (single flute) endmills, and using appropriate router speed and feedrate in the software to get a nice clean cut. Depending on the dimensions of your signs and how intricate the design is, you will need larger or smaller endmills. For sure you will need at least a 1/4" and a 1/8" one, these are by far the most common ones.
Check out those threads for examples of plastic signs:
People also use drag bits for engrave the surface of transparent plastics, like this: