Different ways to approach this.
To add to 6_pac's advice about shared folder by default being shared.
Let's not get confused. The LW Library window shows two folders which are psuedo folders, ie: they don't really exist on your computer but are shown in LW to help you know which files were recently downloaded (Saved Files folder), and files that are individually shared out of a folder.
1. One is you can automatically share your downloads immediately after downloading.
2. You can also allow partial file sharing, which means whilst you are downloading the file, others are downloading parts of the file from you which overall speeds up downloading for everyone. You'd be surprised how much of what you download is actually via partial file sharing. Issue there though is sometimes with large files such as videos, the people or person you were partial file sharing from does not have the last part of the file yet so your download from them stops temporarily. lol This is also 'one' reason why download speeds change around at times.
3. You can also add other folders to share if you wish. See
How to Share Folders (click on link)
Personally I share selected folders from all over my computer. Selected folders from my iTunes Music library, videos from elsewhere, images from another location, etc.