Here are some mistakes you should try to avoid making if you've just signed up for guitar lessons.
Trying to incorporate singing into your playing right away
Many people who have guitar lessons choose to do this because they would like to be able to perform not only the melodies of their favourite songs but also the lyrics. Whilst there is nothing wrong with having this type of goal, it's best to wait until you've had quite a few lessons with your guitar instructor before you start to incorporate singing into your playing sessions.
The reason for this is that guitar playing on its own requires a considerable amount of multi-tasking; you must remember the chord positions, the chord progressions and the fingerpicking or strumming pattern whilst also ensuring that you smoothly transition from one chord to the next. After several lessons, you'll find that doing all of this simultaneously starts to come automatically, with far less effort or conscious thought. However, if you try to learn to do all of these things at once whilst also singing (which requires remembering lyrics and ensuring that each line coincides with the appropriate chords), you'll probably take far longer to develop the aforementioned skills you need in order to enjoy using this instrument.
Not being open-minded about trying the guitar-playing styles your teacher suggests
Like many people, you may have chosen to learn the guitar because you enjoy a particular guitar-playing style and want to master it. Whilst this is a good ambition to have, it's important not to stick rigidly to this one playing style during your lessons and to instead be willing to give the other guitar-playing styles your guitar instructor offers to teach you a try.
The reason for this is that if you decide to only learn one style of guitar-playing (such as, for example, blues style), you'll then only feel comfortable and proficient when performing a specific subset of songs and you'll be missing out on acquiring additional guitar-playing skills that will enrich your playing experiences and make your performances more enjoyable for your future audiences to listen to. For instance, if you tell your guitar instructor you only want to learn how to play the blues after they have offered to also show you the finger styles typically used in classical guitar music, there will be a vast selection of beautiful classical music pieces that you'll either struggle or be unable to play.