Like what are some differences, how much harder is it, etc.
In my experience, it was a rough transition. Classes were harder, but that was the new standard and expectation. Many friend groups crumbled and new ones were formed for strategic reasons. People became very serious and increasingly competitive. Constantly shifting loyalties and alliances as well…
I don’t mean to scare you or anything. Perhaps my high school was just super competitive.
My transition was a little bit rough (due to me moving, leaving middle school friends behind, being shy/depressed). The difficulty of high school classes isn’t crazy, but you should start to build study habits now. Different schools have different levels of competitiveness (mine is so so). Just focus of being open minded, don’t be petty, and focus on classes/try new things. I’m a sophomore now and this year was MUCH better than freshman year
Another difference is,the size of the building and number of students. Even in a larger high school, it is bumped to bumper students. Getting from class to class is difficult. Getting into your locker to change books can be really difficult because you can’t get near it, much less open it.
In middle school, students seemed more social.
In high school, watched in fascination as students texted others when sharing the same table with fellow students. High school students drive erratically, often like they own the road. There is a greater allegiance to the high school and athletes.
So I don’t have that much experience with high school being only a freshmen, but for me personally classes do get harder so just try to put more effort in school and take school more seriously. But my advice is to try your very best but you don’t have to stress yourself out just try to constantly improve and better yourself. And also remember that freshmen year year is your year to adjust to being in high school so don’t sweat it 
My experience was kinda dramatic because I came from a private school that I went to for 9 years to my local public high school. Overall, academically I didn’t struggle at all, it was just the social aspect since I’m very shy. The academics are harder but if you know how to handle your time and study it won’t be that hard of a transition. There are a lot more people and a lot of different kinds of people with different backgrounds and academic levels. Just have an open mind and it’s fine
I agree with all the comments. Don’t panic if your grades aren’t as high as you’re used to. My son was an excellent student but got an 80 in honors science in 9th grade! He went on to get accepted to schools like Wash U and Amherst because his grades improved, 10th through 12th grades.
There are more EC options, so you can try new things. That said, they tend to take more time in HS (bigger time commitment). So try something new, but don’t get overextended so you can keep your grades up.
As a high schooler, you got more freedom to pick courses. When I was in middle school, the only advanced courses you could take were Language Arts and Math. However, when I was in high school, freshmen were open to honors sciences and social studies. In addition, you get TONS of ECs if you’re interested in something such as Forensic Science or Music Theory.
One thing that’s common in middle school is they will have “teams”. If you don’t know what I’m talking about, they’re a group of teachers that everyone else on the team has. Every student is assigned into a team and teachers will collaborate with one another so that students don’t give too much homework. However, high school teachers don’t talk to a lot of other teachers outside of their area. This means if you take many hard courses and have one night where you end up with a LOT of homework, tough luck.
High school also seemed more crowded. My hs had 2000 kids while my middle school had 1000, so I made a lot more friends when I entered the school.
If you have any other questions, feel free to let me know!
Select your freshman carefully. Either you or your parents should look at charts or progressions per subject - sometimes at some schools if you start in the wrong class you’re on a set path and it’s very hard to change.
Carry books and notebooks for several class periods in your backpacks as it’s unlikely you’ll have time to go back between each class. You’ll get a muscled back and arms 
or start practicing now.
Your day will now be extended to include after school activities.
You’ll need to learn how to use your planner. It should become automatic to write in it each period and check it first thing when you get home.
A big difference is that until now whatever you did had no larger consequences down the line. Now, everything matters when it comes to deciding your college choices and majors. Freshman year you’ll still get a bit of leeway ie., some colleges will downplay any problem and some won’t count it but equally as many will do neither thing.
Make sure you are taking five or six academic classes - English, math (algebra 1 or geometry depending on what you took in middle school), foreign language level 1or 2, a science (biology, chemistry, physical science, or living environment), and history or social science (AP human geography, global history honors, etc) plus one more class (such as art or music). Often your school will tack on Health and physical education.
Talk with our current teachers to see if you can qualify for the honors version for each class - it’s okay if you don’t but it would’ve too bad if you is qualified and didn’t know about it.
Be disciplined about sleep. You need to get 9 hours of sleep minimum to keep your brain healthy and functioning at top level.
It means relinquishing your devices at 9pm.