On Windows 7 (I'd imagine it's very similar in Win10)
- Right click on the folder you want to share and select properties
- On the"Sharing" tab, click "Share" and you can add just yourself or everyone
- If Everyone is added, you can set the permission level to read only or read/write
To access shared folder from computer B, open file manager and type \\ComputerA\
Replace ComputerA with whatever name you've given it or use ip address, you might need to enter password.
You should see the shared folder. To make it simple the next time, right click on the shared folder (still in the file manager of computer B) and select Map Network Drive. Give it a drive letter, for example X:
In the future, you can just click on X: drive to access your shared folder and it's contents.