I have a new LG G3 and I want to listen to music that is saved on my SD card. How do you do this?
Your phone should have a built in app (Google Music, Music or such) for music playback and they usually auto-detect any music on your phone (in any folders you can access). Keep in mind that there is no "best" app out there - they all have flaws like not having 100% effective bluetooth control or consuming more battery with equalizer.
I've tried some of the most popular ones, but returned to default one, because of lack of bluetooth forward/rewind support on most players.
Default music app (whatever it might be on your device) is usually gives you everything most users would need.
Winamp (unfortunatelly no longer on google play) boasted a good equalizer, but quite frankly it doesn't compare to n7.
n7 Player is somewhat uniquely designed and has decent equalizer options, but equalizer produces delay between action (like song switch) and playback. Also it seems to somewhat increase battery consumption.
double twist player has a lot various expansions and online functionality however, most of those features aren't free.
Poweramp Player I've been reminded of this one in the comments. It did not catch my attention, however, at the time I was checking out it WAS one of the highest rated ones.
Rocket Player I haven't tried this one. However seeing as it is free and has decent ratings it might be worth checking out. As for the playback, if you have music on your sd card, open up your music app and the music should be there.
If there is something preventing you from listening to your music please specify it.
There are two ways to do it. The first method is to connect the SD card to a computer and dump the music in. From there you can use any app that supports audio files. The other method is only if you have music in your Google Play library. You can add music by purchasing or by uploading to your music library. Once you have done either (or both, whatever), go into the Google Play Music app on your phone, tap settings and click storage location. From there set it to external (when you have inserted the SD card into your phone). Once you have done that select what album you want to download and hit the download button, which is an arrow pointing downwards, hitting a vertical line.
If you are taking about microSD, the pop them into the card slot and the let Google Music or other software as mentioned by @zero discover. If its SD card, then you should use a card read to connect SD to a PC and then copy them into your phone.