I'm sure that Brink has already made a tutorial about this but I just finished installing Win 10 and I had to share a folder so i documented it.
1: Right click on the folder you want to share.
2: Select Properties
3: Select Sharing
4: Select Advanced Sharing
5: Check the checkbox for Share this Folder
6: Ether change the Share name or leave it as is.
7: Select Permissions
8: Click on Add
9: If you want you can safely remove or un-check all items in the Everyone (All Users)
10: Type your Microsoft ID (email)
11: Press the radio button "Check Names"
12: Press OK
13: Select all Allow checkbox's for MS Id.
14: Press Apply
15: Press OK
In step 9: and 13 if you select the checkbox Deny that will deny sharing for all Users.