I have no motivation.

[Danascot]

7. Just do the first step

If that's going to Lowes to get screws, do that. Assure yourself that's all you have to do at the moment. Be satisfied you got that done. Then the next day or two days or three days do one more step. At some point you may feel you can do another step without pausing but it's fine if you don't. Keep doing one step incrementally at the pace that you're comfortable with and eventually the task will be done.

This is in addition to the suggestions to watch youtubes about building shelves.

Having no motivation is a sign of depression. It sounds like you already know that but I wanted to mention it in case you didn't. I suffer from depression. Fortunately I've been on medication for about 30 years that knocks it down to a more manageable level.