Tuning our instruments is a high level skill and beneficial to a worthy practice session.
Please watch the introductory video, then move on to your specific instrument's video.
You may use any tuner that you wish, but here are the ones that we recommend:
Apple devices: ClearTune (free), TE Tuner
Android devices: da Tuner Lite or gStrings (both are free)
SmartMusic tuner (if you are a subscriber)
Snark clip on tuner (if you already own one)
Look for Tuner Apps which have a rating of at least 4.0 or higher for reliability.