You don't need `--sourcepath`. It looks at where you are pointing and tries to pull items which match what you have there already.

What you do need is a `package.xml` file to get your entire metadata database; you might need help getting that file for your particular org. 

Instead, you can do the following to get going with Apex or Lightning Component development:

```bash
force:project:create -n MyProject
cd MyProject
sfdx force:source:retrieve -m CustomObject
sfdx force:source:retrieve -m ApexClass
sfdx force:source:retrieve -m ApexPage
sfdx force:source:retrieve -m ApexClass
sfdx force:source:retrieve -m LightningComponentBundle
```

That will populate your source tree, and `--sourcepath` will work when it's pointed at one of those folders.