For example I have a SharePoint library wich I want to access through teams:

1) Find the URL of the SharePoint library: https://keminind.sharepoint.com/sites/ServiceCatalog/AllKeminFlowsLibrary

2) Open the TEAMS where you want to add this library and click the + (Add a tab)

3) Select the "Document Library" app.

4) Click "Use a SharePoint link

5) Paste the Library Url from step (1)

6) Click Go 

7) and click Next

8) Pick the document library from step (1)

9) Click Next

10) Give your tab a meaningful name

11) And click Save