how do I sync selected songs from my windows 10 computer to my iphone

I don't think it's possible to select individual songs as such, but you can either create a Playlist and Sync that, or select albums, artists or genres.

• Once you decide, connect the device to iTunes and click on the Device icon:

• that shows you the Device/Settings/Summary pane
• switch to the Device/Settings/Music pane (1.):

• tick the box named Sync Music (2.)
• in 3., choose Selected Playlists, artists albums and genres. Only now will you see the sections in 4.
• choose the items you wish to sync from section boxes 4. If songs is listed, it will be further down that screen, but I'm pretty sure it isn't there
• if you've made a Playlist, choose that. Otherwise, select the albums etc. that you want.
• Click on the Sync (or Apply) button in the lower tight corner of the window (not shown in the screenshot)

One other possibility; if you have Sync only ticked songs and videos selected (on the Device/Settings/Summary pane), you can then untick (uncheck) any song in the Library and it will then not be added to the device. This is a clumsy and cumbersome method, I don't recommend it.