Step 1: Pick which of the three races you want to learn first.
Since each race plays mechanically different you’ll make a lot more progress if you stick with learning just one race and getting the mechanics of that race down first before attempting the others.
Step 2: I’d recommend finding some YouTube tutorials, probably a B2GM (bronze to grandmaster) series where you can usually see what skills it takes to play at a certain league’s skill level, and those series usually teach you beginner build orders too.
This should give you a good idea of what to work on.
Step 3: Practice executing your build orders on the easy AI’s in custom games (you usually want to start with BOs centered around a specific timing attack so you have a clear goal to aim for, like hitting your timing faster and faster as you practice).
This is my preferred method of practicing, because usually the easy AI will give you plenty of time and room to do your build, especially if you set their strategy to defensive or economic.
Step 4: When you feel confident enough in your timings and mechanics why not hop on the ladder!
You will most definitely lose the first few ranked games until your MMR evens out and gets to your actual skill level, so don’t get discouraged!
And most importantly: have fun!
Edit: there are tech trees for each race too, you should look them up if you think that will help visualize how the different “tiers” of tech are set up for each race.
In SC2 the equivalent of aging up is based in building buildings rather than researching something in your town center to get to the next tier of units.