Here are some options:
Online:
https://yousician.com/guitartuna (the famous guitar tuna app) - the old joke you can't tune a fish, but you can tune a guitar 😄.... - you can also download this on your mobile device Mobile (your iPhone, android, etc.):
Like I mentioned up above the Fender app and the Guitar Tuna/ Yousician are my two favorites.
The thing I like about the Fender app is that you can tune to Drop D, 1/2 step down, open tunings, etc. - more about this to come later if your new to all this, and your not 100% sure what I'm talking about yet.
The Guitar tuna app - the thing I've noticed is that it will give you a check mark and sound (if it's turned on) if your pretty close either flat (below the note) or sharp (above the note), so I have to pay really great attention to make sure the line that's telling me its in tune is on the middle line exactly.
There's others I've tried as I wasn't as impressed with... comment below with what you use or like if you have a preference 👇
You can also get a lot of these apps on your tablet depending on whether it's a Kindle (android), or iPad (apple), etc.
If you would rather use an actual device that you can put on your guitar (I did this for many years).
I would use this one (many variations now):
The Snark - weird name but works great! Highly recommended!
I remember using a watch battery for this one for years, now you can just charge it via USB.
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Let me know how you tune your guitar by commenting below: 🎸👇