i have windows 7 can i convert songs in my library to mp3 (then on to an sd card)

Yes, as long as these are songs you purchased from the iTunes Store or imported from a CD (for example). You cannot convert songs you leased with an Apple Music subscription (they are copy protected). Fo more info, see:

Convert music file formats in iTunes on PC - Apple Support

or

Convert a song to a different file format with the Apple Music app or iTunes for Windows - Apple Support

Also see the reply to:

Can't convert purchased song to mp3 (iTun… - Apple Community

Yep, you can download songs with a Apple Music Converter on your desktop, then use a USB cable to put them on your SD card.