As a new player without having played any rts for a really long time, the first thing I had to learn to beat the medium AI was just the basics of super-early macro (e.g. not spending all ur minerals on random stuff). Get a supply depot out, then a refinery and a barracks, then second supply and then a 2nd command center.
There are more intricate concepts I’ve been playing with that I’ve seen a lot, such as getting a tech lab as soon as you build the barracks, research stimpack, then after building a factory, switch them over, build a reactor and bulk marines. After a couple of medivacs you can just spam tanks etc. Against the easier AI, you can more or less just do this and then storm their base with one substantial all in attack. No 4th bases, no harassment, no real tactics, but also no real micro.
I absolutely suck at micro. When I try to play vs hard or when I try to be a little more active, I always just seem to get fkd. Hellions die as soon as they enter the base to try harass, reapers die before they pick off any workers, I lose a majority of my army to roaches or ultralisks, my medivacs always seem to get eliminated almost instantly in any fight due to their movement speed over the marines, and any tanks or ghosts I may produce just get picked off before I can control them amongst the mess. How tf do I get better at the micro? Do I just need to be using more assignable hotkeys? I swear I use one for the infantry, another for the ghosts and another for the tanks, but whenever I do this I still just lose track of everything before I can do anything significant…