Step 1: go to your hotbar and scroll to the thing that looks like a weapon (or press 1) (use the left joystick for console)
Step 2: now that you have the weapon highlighted, use the left mouse button (idk the thing for console) and click.
Step 3: watch as your character uses the weapon you clicked.
Step 4: if its not working, either you did not follow instructions correctly, or you are either out of mana or ammo.
Step 5: if you're out of mana, wait for it to regenerate.
Step 6: if you're out of ammo, forage and find the ammo for that specific weapon.
Step 7: one done all that, repeat steps 1-3.

If you followed all the steps correctly, you should have used a weapon! now use it multiple more times to defeat everything in your path!