You will need a tuner in order to tune your guitar strings to the correct pitches. It’s also important to note that you will need to continuously tune your strings right after replacing them, as they will need a bit of stretching out. The best way to do this is to just play normally for a minute, stop, tune up, and repeat until the strings stay in tune.
There are a few different tuners you can consider using, and they all depend on what your personal preference is. There are handheld digital tuners that you plug your guitar cable into, or there are clip-on tuners which clip to the backside of the headstock of your guitar and don’t require any cable to use. Some handheld tuners also may not require a cable if they have a built-in microphone, which is useful for tuning an acoustic guitar. Foot pedal tuners are on the more expensive side, but are essential for those of you who want to delve into the world of guitar effects and pedalboards.