I am not that much of a 3D guy myself but watching modelling sessions for advanced models and seeing which tools they use is a good practice for making shapes. To me imagining art is always diffrent. For most of my life ive been drawing random pictures, it got easy to imagine them and draw the. But when i transeffred to pixelart and wanted to make an example image in my head it was hard. It takes time for your brain to adjust to a new art type, at the beggining i was unnaturally bending pixels in my head and would be dissapointed to see the out comes but overtime i learnt to make an image for pixelart in my head. For now id reccomend getting comftroble with the program and exercising imagining a weird drawing in 3 dimensions. Id also reccomend practicing before you start working on the game so you do not waste hours of redoing the place holder assets (Speaking from experience). But yeah practicing the convert from the brain to the program is probably the way to go for you right now.