I'm a prep student in an all-German school and I really wish I had some help before I started so here's the help you need-from a pro!
1-DO NOT FREAK OUT!
2-TRY TO SPEAK EVEN IF YOU SOUND STUPID
I have been a speaker for a while now and even now some of my teachers don't understand when I say stuff. It's okay to have problems with "z" and "s" and "t" and if you are an English speaker saying every letter in the word will be impossible at first but try. Use google translate to listen to the words, then repeat it. Raise your hand in class and talk, you will be shy at first because you will doubt your skills but you have to practice before you can be even okay at the language.
3-FLASHCARDS REALLY HELP
3-FLASHCARDS REALLY HELP
I thought it wouldn't but if you make them yourself, if you read as you write and if you use a dictionary to check out the meanings at the same time, you will learn them by heart. After that, just shuffle the cards and look at the language you used for the definitions and try to guess the German words, don't do it the other way because your mind will be translating from your native language to German instead of translating German to your native language most of the time and if you can do the hard one, the easier will already be done.
You will not learn the grammar topics the first time you hear them and it really won't help to re-read your notes a billion times because you need to see its usage in the language or it will be just in your memorisation and will be useless. Do your homework, take every single piece of example you can get and do the exercises and tasks.
5-THERE WILL ALWAYS BE A NERD, DO NOT MIND THEM!
There is a dude in our class who reads the fucking German dictionary and who spends literally 5 hours each day studying, and yes "that dude" will do better than you in the class and in the exams. Don't mind "him", there will come a day when "he" won't be able to study because the answer won't be buried in the studybooks. Just be busy with your life because "he" is going to be useless one day and you will be useful!
6-DO WATCH A LOT OF GERMAN MOVIES AND TV SERIES
You probably will need to use the language in real life instead of memorising stuff and forgetting them the next day. This is how I learnt English and now I'm correcting my teacher which is why I know it's useful. When you watch a show in German, you will learn the daily phrases and simple words they use and things they never say or the "slang" language. Your teachers tell you how to say "I'm going to" but you won't know the phrase "I'm gonna" unless you hear actual people speak, and TV series is the best way to do so. Once you can understand at least a little bit, you will start learning from it too...
And it's a big AND I MEAN BIG confidence boost to understand at least a few words from something you're watching that isn't for kindergarten students.