The traditional and most standard tuning for guitar from the lowest (thick string) to the highest (thin string) is E A D G B E.
????Tip - An easy way to remember the string names is with this handy saying: Eddie Ate Dynamite Good Bye Eddie.
Tuning with an App
There are many apps available for free and purchase on iOS and Android platforms. My personal favorite for Apple devices is InsTuner.
????Tip - Make sure that there isn't too much background noise when using an app-based tuner because it gets easily confused. This goes for any over air tuner. The closer you put the device's microphone to the guitar, the better it will pick up your instrument.
The most convenient tuners right now are clip-on tuners. They attach to the headstock of your guitar and allow you to tune even in loud environments because it uses the vibration of the instrument as opposed to sound traveling over the air. It works well for both electric and acoustic instruments.
The most popular version of clip-on style tuners you might see is the Snark, however, it doesn't tend to hold up well to regular use and often brakes at the pivot joint. I've had way better luck with the Korg PitchCrow which is a bit more expensive but is way more durable and accurate.
Tuning by Ear
Tuning by ear (also referred to as reference or relative tuning) is done by using another instrument or the guitar itself to tune each string. It is most useful when you are playing by yourself or if you are playing with another instrument that is out of tune like an old upright piano.
Here's a great video tutorial to walk you through a few different methods of tuning by ear: