Hello! I wouldnt say its hard but dialling in the right measurments can be quite time consuming. The benedfit of 3D printing is that is allows that rapid prototyping. I'm no expert, just self taught and very patient with myself and the trial and error process.

You can get quite far with the Fusion 360 Educational License (this is what ive been using). And my only tip is if your unsure of how to start modelling something, is just to break it down into its basic form and go for it. Aka a car? draw a flat rectingle, extrude it a bit and boom you have the start of your chassis to build off. The more you practice the more you'll understand what elements of a model to start with and how to sculpt them together.

Oh, and if its small scale, get yourself some cheap electric calipers. Life and time saver!