Well, first of all, WELCOME TO DEVIANT ART!! a great place to find advice, competition and good art ^^
I see you want to get into art and design!! great!! I´m looking to that too hehe
So we´ll be improving together!!
Hmm, to create a good portfolio you´ll need first of all focus on drawing tons of sketches!! Don´t think of drawing finished products yet, leave those for later. Or if you want to make some, don´t abuse, just get focused into practicing with sketches! those will give you the ability to play with form and art itself!! really, you don´t have an idea on how that can make you improve in almost no time!! But you need to be quite consistent and decided to take hours and hours fighting to solve poses, learning anatomy, getting some painting tips, testing mediums, looking for programs, studying tutorials and getting other artists information. Well, I think that´s enough. You may find lots of things by yourself, so don´t stand there and look for things and sketch a lot!!
I´ll give you a deviantWatch to see how your art improves!!
See you ^^Ohh, and thanks for that watch