I can't believe how many people have SD Cards slots on their phones, and don't use them. Better yet, I can't believe how many people have an SD Card inserted into that slot, but still don't use them because they don't know how.
A lot of people just assume by putting an SD card in your phone, that your phone will download to it, save music to it, save photos to it, etc. You have to manually select these things in order for that to happen. For example, in the camera app on phones that contain an SD card, there should be an option to save photos to "External Storage". Make sure that is selected, rather than "Internal Storage".
If you download music, 99% of the times it's being saved to your Internal Storage. You will manually have to move these music files to your SD card (that is until you set the default save location to your External Storage). My suggestion is to make a folder called "Music" on your External Storage (SD Card). Save all your albums in there, and make sure that is the default folder to save all your music.
I try to keep as much as possible on my External Storage to free up the space on the internal storage. This will keep your phone nice and quick.
Another tip, clean your Internal and External Storages for unused files. I usually go through them every couple months, deleting old music, photos, etc that I don't want anymore. It's amazing how much useless stuff we keep on our devices that we don't need after a month or so. This will also clean up some free space and have your phone loading quickly.