I'm going to start music lessons this summer, but I'd like to know whether I could learn something upfront. I'm a complete beginner and don't know what I could do? I installed gnu-solfege but don't know exactly what to do with it? What's something worth learning for an absolute beginner?
If you are a complete beginner, GNU solfege may be a bit much at first. As one review on their site reads,
However, after testing the program, you can easily tell that you need to have already some basic – if not intermediate - music knowledge if you want to make the most of GNU Solfege.
Nonetheless there are many other places you can visit that will help you. Here is a good site: https://www.musictheory.net
If you have access to a piano, learn what the notes are on the keyboard, and then play some scales (maybe just stick to the treble clef at first). Here is a site that shows basics about scales: https://www.pianoscales.org/
Also, importantly, try to find a friend who is knowledgable about music. They can show you firsthand things that you might be missing.
When you practice, be sure that you go slowly and accurately. Practicing too fast is counterproductive.
The best plan (apart from the very good advice about listening!) would be ask to your teacher.
If he/she starts complete beginners on practical work with their instrument, and lets the theory follow on from that, the answer might be "no, there isn't really anything much that you can do before the first proper lesson". Some instruments need minor adjustments to match the player (e.g. chin rests or shoulder rests on violins, slings and straps on wind instruments or guitars) and if those things aren't right for you, then you are be starting off by using the wrong body posture - which is not a good idea!
Learning music is like learning to ride a bike. You won't learn much just by reading books about the theory of bike-riding - you actually have to do it, and fall off a few times as you learn.
If you try to start playing an instrument without any tuition, you may be developing bad habits right from day one, and you will then have to unlearn them - and that might take more time that if you just waiting! Without any guidance, you can't tell whether a slick-looking "how to play the [whatever]" series of YouTube videos is teaching you good or bad habits, or whether you are looking at material for a "complete beginner" or somebody who is more advanced. There are a lot of good tutorials, but there is also some complete garbage - just as with everything else on the Web.
I find that it has great worth for musicians to first discuss their ambitions with their new teachers and to also share their musical likes with their teachers.
If you are a singer just take your MP3 player with you and play some of your favourite music for your new teacher. Do you want to be a opera singer or do you want to do musicals, some insight into why you are starting with your music lessons may a worthwhile way to start a teacher-student relationship?