There's no right or easy way to do this. Unfortunately, sometimes we hit patches in life where we have to do something so stressful and hard and scary and there's just no easy button to hit to make it all go away. You're in a really really tough position. If there was a way to make it safer, I'd do that for you because nobody deserves to deal withe the stress and pressure of an unplanned pregnancy. But it's also great that you're choosing to not make yourself deal with this alone.
Now, onto the advice you asked for! Even though you're freaking, try to remember that he's clueless right now. When you tell him, he's going to have to deal with that initial shock that you had after taking the tests. Try to sympathize with that by staying calm when you first tell him. You can freak together later.
I definitely suggest planning when you can tell him everything. Do it when you know that neither of you has anything to do for a while. Then you can have time to talk and support each other. Also do it somewhere private so you know you won't be interrupted by others. After that, all you can really do is tell him straight out that you took tests and you're pregnant. There's no other way to say that. What's important is that you try to make the setting private, quiet and intimate. Then he can feel free to express himself to you, and you can feel free to express what's going on for you too. Really, just taking the time to make it as calm and possible will go far.
After that, don't be scared to be honest with him. About freaking and being shocked and scared and even upset. I'm quite sure he'll be feeling the same way. Just give him those first couple minutes to get over the initial shock. Then don't hold back, he's in the same boat as you. He's dealing with the same issue and will surely understand everything you're feeling.
He's your partner in this, you two have to stick together. Be open and talk together, neither of you has to handle the stress and worries alone. You have each other to lean on, don't forget that.
I know this is hard, but you can handle this. And after you tell him, you won't have to deal with everything alone anymore. And you can stop imagining every horrible way he might react because that will be over with, you'll just have someone there who understands the difficulty of being in your shoes right now.
Good luck, I hope you get a good reaction. And we're here if you need anything. Just plan as much as you can and blurt it out, get it over with instead of letting this pressure build and build.
<33 Audrey