I can't do a split screen on a dreamweaver CSS file? ( I have never worked with dreamweaver before)

Before you go any further, you first need to create a Site Definition,  so DW knows how to handle your site's assets. 

It'll be the first step every time you create a new site with DW: https://creativecloud.adobe.com/cc/learn/dreamweaver/web/define-a-dreamweaver-site 

Without that step done, you will run into all kinds of "this isn't working right" style issues.

JavaScript, CSS, XML, etc... are editable only in Code View. 

HTML files can be opened in Split View as shown below.

For best results, work with valid code as denoted by the green checkmark in a circle below the document panel.

A red X indicates the code contains errors.  Window > Results > Validation.

CODE TUTORIALS:

- https://www.w3schools.com/html/
- https://www.w3schools.com/css/
- https://www.w3schools.com/js/

- https://www.w3schools.com/bootstrap/

Hope that helps.

thanks ! but how do i even connect my css file to the html one?

1. Open an html file that has your CSS file attached to it
2. Go to Window > Toolbars > verify Document has a check by it
3. In the Document toolbar, verify you have Split highlighted. This will show your code next to either the Design View or Live View of your page
4. Click the small triangle in the Document Toolbar to select Live if it's not
5. Now you should see your code, next to a Live View of your current html page
6. To work on the CSS attached to the page, click the .css file name in the top of the document window, just under the named file tab. When you make a change there, it will instantly update in the Live View. ...

7. When you're satisfied with the CSS, you can hit Ctrl + S to save it, then click Source Code in the top of the document window to go back to working on the html page itself.