All five classes in Diablo 4 have a myriad of skills available to them, organized across a system of different types and clusters, so, at first, it can be a bit hard to figure out what skills you should even pick and, subsequently, how to use them.
Though skills are assigned to their respective standard inputs automatically if you follow a one-skill-per-cluster pattern, if you happen to like switching it up and going against the grain, and you've picked two abilities from the same cluster, then it can be hard to figure out how to use them.
How To Assign Your Skills
What you must first do is open up your character menu, then scroll one tab to the right to the Abilities page. From here, you'll see all the skills you've trained up with your skill points, and you can get a peak at all the future skills that will be available for you to learn as you level up.
Looking at the bottom right of the image above, you'll see that there's a button prompt for skill assignments. Clicking this button will open a menu that displays all your learnable skills, with the skills you've already learned bright and vivid and the skills you haven't learned grayed out.
From here, all you have to do is simply select a skill you've learned, and you'll have the option to place it on any one of the six available button inputs for abilities. If you're a Sorcerer, as seen above, you'll also have the option to place skills in the two Enchantment slots, which give you bonus passive effects that vary depending on the skills selected.
Note that if you're still in the early stages of the game, you will have locked-off input slots that you won't be able to use, even if you have learned enough skills to fill them up (as opposed to rank up two or three skills). You'll have to progress the game some more, level up, and then you'll be able to mess around with your build as you please.