Below are a few helpful tips to keep in mind when you start taking guitar lessons.
Adopt the same position you'll need to be in when you eventually perform
When having a lesson, you should ensure that you adopt the position (i.e., either play seated or standing) that you will need to when you eventually perform. For example, if after becoming a good guitar player, you'd like to join a band, become a solo artist or start busking, you should play your guitar whilst standing up. Conversely, if you're learning classical guitar or if you just plan to play in your own home for fun, you can stick to having your lessons in a seated position.
The reason for this is that if, for example, you spend all of your lessons playing this instrument whilst sitting down, you may struggle to adjust to playing when standing up when you are eventually asked to do so. This is because you will be used to most of the guitar's weight being taken by your legs, rather than your back (via the guitar strap) and because sitting down makes it easier to curl over the guitar and closely view your finger positions and strumming movements.
Likewise, if you have your lessons standing up and then pursue a guitar-playing activity that involves sitting down (like classical guitar, which is typically performed whilst seated), you might find that you develop sore spots on your legs where the guitar's body presses into them.
Tell your guitar teacher what type of guitar music interests you
Many students who take guitar lessons let their teachers pick the music and just go along with whichever genre the teacher prefers or recommends. However, you do not have to do this, and in fact, you will probably gain more value and pleasure from your lessons if you are forthright regarding your guitar music preferences. Aside from simply making the lessons fun, telling your teacher what music you like will mean you'll get to learn the guitar-playing characteristics of the genre you love.
For example, if you enjoy blues music, you and your teacher can focus on this genre, and they can teach you classic blues licks, how to do a slide and how to master the shuffle rhythm. This will enable you to begin playing the blues music that you love within a few months of starting your lessons rather than having to wade through months of playing music you find dull before getting to do this.