You download the Minecraft_Server exe from the official site, place it somewhere and then run it once then after it finishes generating a world see if you (and other people) can join it.
If this is not the case open server.properties in notepad and edit the port number (server-port) or port forward the default port Minecraft uses. If you went with a custom port you will have to port forward that one instead.
Remember that the server application has to keep running for the server to stay up and it will probably slow down your computer significantly.
To let others join your server you'll need to give out your IP, and if you used a custom port, the port too. This is done in the format IP:PORT so for example 123.456.7.890:5353
(I typed this from memory, information may be wrong but I think this covers it.)
