Guitar accessories cannot replace a well-made guitar, and they are not a replacement for improving the techniques that comes with practice and taking guitar lessons.
There are some essential accessories that it is hard to think of them as accessories.
Guitar Strings & Picks
For example a complete set of extra strings and picks. Guitar strings break at the weirdest times with no apparent rhyme or reason. And guitar picks have a habit of getting lost regardless of how careful you are.
You may be able to get by without a pick and practice your finger picking techniques, but a broken string could stop most in their tracks except the most avid and versatile pros. I have to admit that I am not one of those pros. I always have at lease one set of complete strings and when one breaks I replace that one.
Guitar Tuners
As a beginner, you need to work on training your ear as much as your hands. Ultimately guitar music is heard and felt. A tuner can help your guitar provide a perfect pitch and your ear to recognize that pitch.
Amplifiers & Cords
For electric guitars, an amplifier and the attachment cable is not an accessory and as you practice you would notice the connections may not work as well. A cleaning solution becomes necessary.
Guitar Cases
Guitar cases are another accessory that beginners may not think of until they get the first scratch on their guitar. Even if you are super careful, guitars don’t necessarily do well being exposed to extreme weather conditions and humidity. A guitar case also provides protection against elements.
Obviously a hard guitar case with soft material inside offers more protection. If you invest in a hard case, look for how the pockets for the accessories are set up. You can keep your extra set of strings, your picks, and even cords in those pockets for easy storage and reach.
Straps
Straps is another must have. Most beginners usually sit and play as they learn. If you start playing in a band and on stage, the posture is different, and it may impact your technique. It may be a good idea to start practicing standing up using a strap.