>>5981018For an absolute beg, I'd start with basic intro courses somewhere like Schoolism (I believe they have a sale on year long subs in May) and start building your art skills in general.
Once you're at a comfortable point with your general art skills, definitely study golden age illustrators like the anon above said. Focus on composition, value, depth, mood. Some great books to read once you're at a good level of skill are Framed Ink by Marcos Mateu-Mestre and Color and Light by James Gurney.
Check out some current MTG artists that show thumbnails and WIPS to get a sense of what their process is like. See if any artists you admire have classes available. Try checking out the Workshop Academy, some WOTC artists offer classes occasionally.
In addition to all of this, if you really want to start working in games you'll want to make a Twitter and get a little involved in the TTRPG community. Post your progress and follow other artist interested in the same career as you. There's a ton of D&D fan books that are always looking for artist of any caliber to work with them and sometimes you can get some pretty nice dividends from sales. Practice the work you want to do on a smaller scale and it'll also help you get used to a similar working environment that you'll get working at a bigger card game company.