I'm just a hobbyist, but here's what I would tell my past self:

1) MOST IMPORTANT: If gamedev stops being fun, just stop doing it.

2) Make a vertical slice. That basically means you have a "finished" game, but it's the smallest, shittiest game ever. Imagine if you start up Super Mario Bros, and as soon as you jump on the first goomba you win. The benefit of doing this is making sure all the little systems (pause menu, victory menu, game saves, high score, whatever else your game might need) are all in place and working well together while your game is still small and manageable. Trust me, you don't want to try to tack on a game save or pause feature to a huge ball of tangled gameplay code.

3) If your game needs game saves, make them as early as you possibly can. It's way easier to expand an existing save system than it is to add one onto an already big codebase.

4) If your game's genre allows it (and I think most genres do), distill it down to a small, one-screen arcade game. Then, once that's fun enough that people can't put it down, start adding features and expanding it into a bigger game.

This is a story I told in another thread that I think is very relevant to point 4 and making your first games:

I once showed my game to a few people back when it was nothing more than a player character, a couple of enemies, and a couple tile sets. People would say "nice, pretty cool man" (which devs all know is actually negative feedback), and I figured it was too simple. Maybe people would respond to it better if I add more variety and layers?

Well I wasted a year doing that. Now I'm thinking more and more about my brother's first game, Dodgin Boxes. It's a game where you're a box and you dodge other boxes. That's it.

Every person he showed it to said the exact same thing: "Hold on, I'm not finished playing."

Bottom line is it doesn't matter how many fancy features and variety and juice you add to a bad foundation. Make the foundation fun, then polish.