If you're good at traditional drawings, try getting into digital inking.
Basically scan a drawing, then trace it in a drawing program with more refined lines. This will help you get a feel for the motions needed to draw in the program.
I'm still a novice but here.
https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/intermediary/f/5687b2f8-8af7-41aa-bdea-cd615c6ef5bf/dcw2sbw-7d0a08a6-67fa-4b22-ab39-05d2010bc914.jpg/v1/fill/w_600,h_800,q_70,strp/lux_and_ezreal_pool_party_by_terozu_dcw2sbw-fullview.jpg
This is an image that I inked and colored. It came from this drawing:
https://i.imgur.com/mpHHOZ0h.jpg
This is actually how many comics are made.
This however is fully digital no pencil drawings:
https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/intermediary/f/5687b2f8-8af7-41aa-bdea-cd615c6ef5bf/dcxcn47-8fa39201-a91f-4c83-98fa-f906f7e34b98.png/v1/fill/w_1920,h_925,q_80,strp/star_guardian_wallpaper__by_terozu_dcxcn47-fullview.jpg
While I'll occasionally do original PC art like the above, most of my artwork is inked and colored. And I think it comes out fairly well.
The important part is to keep at it and you'll get better with practice.
As for 3D, assuming you mean models and sculpting, download Blender, watch tutorials. You'll pick it up as you go.
Also worth noting is that any art program can be used with practice. So never judge something by the number of tools it has.
For instance I made this:
https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/intermediary/f/5687b2f8-8af7-41aa-bdea-cd615c6ef5bf/dcw2ve6-aa1fa0e9-5762-42cf-aaed-60f3f0011970.png/v1/fill/w_1024,h_721,q_80,strp/bilgewater_woman_by_terozu_dcw2ve6-fullview.jpg
in MS Paint It's by no means great but the blending in the program is better than most credit it with and it's also good for pixel art stuff. Speaking of which, if you wish to move into pixel art and sprite animations, I highly recommend Piskel.
https://www.piskelapp.com/
It's a great, opensource app, it's entirely free to use and very intuitive and easy to use in my opinion. You have multiple options for saving and exporting projects and everything you make is still your work.