How do I make a song on iTunes into a ringtone?

Hey, so I'm totally new to the whole apple community, just bought my first iphone and figured I'd set a song I like as a ringtone, easy peasy. Or so I thought, since this was such a simple thing to do on any android.

Turns out it isn't as easy as I thought it would be, because reasons. Bought a song on iTunes, but realized that I can't just use it like that, I have to "cut" it down to 30 seconds. Fine, downloaded iTunes on my PC, set the start and end time of the song to 30 seconds, but when I tried to convert it to AAC, that option was greyed out. So following the guidance of the internet, I went to Edit > Preferences and changed the import settings to AAC encoder, 128 kbps. But alas, the convert to AAC option is still greyed out and I've run out of ideas on the internet. Can anyone help me out with this?