College is a whole different ball game compared to high school, it hits you like a ton of bricks just how much you have to manage. I just graduated from a 5 year engineering degree and I had to learn as I went, but I'm a master ball-juggler now haha. It's important to learn these skills now because as someone who's now in the workforce and living on my own, managing multiple things at once doesn't go away when you graduate, and the internet (or worse, the power) will go out if you forget to pay the bills!
I'd suggest getting a planner or using Google Calendar to track your assignments/tests. Your brain's purpose isn't to remember tests dates and due dates. Let your calendar do it for you. I used a bullet journal because it was flexible and cheap, all you need is a blank notebook. Always try to start the assignments early, I used to try to finish them for a day or two before they were due so that I'd have a bit of a buffer. Staying on schedule (or ahead if you can) is one of the biggest ways to avoid stress.
Find a good place to study, this is vital. If it's a certain table at the library, or my faculty gave my class 24/7 access to a lab that we could work in. The important thing it's not at home where you'll inevitably find things to distract you. Always work/study in that area. You'll eventually associate that place with work and it'll be easy to focus.
As for stress, I always followed the rule that unless absolutely necessary, I'd never work on a Friday night. Saturday was pretty relaxed and Sunday I'd start back up with work again. I'd always plan something to look forward to after a stressful week, whether it was a hot bubble bath or going out drinking with friends.
If you can make friends within your classes, do it. My best friends all come from my class. We joke that we bonded through suffering. When crunch time really came around, it was a lot easier to go in the lab on a Saturday morning to work on assignments when my friends were right there with me. We all learned a lot better when we could discuss things too.