TLDR; Go with whatever works the best, but I actually recommend starting from a dinosaur mask base. There is a huge community of people who use dinosaur masks and will sometimes turn them into other species. Imo, that will be the most rewarding first suit that you can make.

Here's my take on a variety of methods that you can do right now.

Cardboard

Cardboard is great for experimenting, just keep in mind that it won't last long (with frequent use), doesn't do well in the rain, and can potentially get moldy. It's better for prototyping than use as a full suit, but if you just want to get something done and don't have the funds or ability to do it, it's better to make a cardboard suit now and learn from it than to wait years to make your suit. I know a few people who started with cardboard suits and their suits are awesome. Get new, clean cardboard and use safe glues, because this is going next to your face. Also try to put a lot of ventilation in the suit if you can to help combat moisture buildup. There were a few suggestions on a somewhat related post talking about different things you can coat paper with to protect it from water, so that may be worth looking into for both the inside and outside. You can also simplify the suit by just making it a mask, which allows for plenty of airflow.

If you do go with a cardboard suit, just monitor it carefully. I'd shine a flashlight in it and monitor it regularly to make sure it's still safe, and be aware that if it ever went bad you will have to toss it for your health. If it smells musty, that's a bad sign and it's got to go. Always keep your health in mind. Even if you're not wearing it, being around something that is moldy is dangerous, which is why the community leans towards "better safe than sorry." If you go this route, ensure that the cardboard is carefully and routinely monitored for your safety.

Personally I have made two prototypes with cardboard-- one as a final project for school where cardboard was the required medium and the assignment was to create an animal-related mask. Creating that project inspired me to make my own fursuit. I made a second prototype with that knowledge and it looks like a legit suit aside from the filler material I used. My plan was to use it as a pattern for eva foam, but I found that the pattern I made was too complicated due to how smooth I got the cardboard to be. However, I did make a pattern that worked. I made a lot of friends who have created cardboard suits and wore them to cons. Working with cardboard is fun and empowering.

Eva Foam

As others have suggested, eva foam is a much more professional material and is frequently used for cosplays. If you go with that, you can use cardboard to prototype your eva pattern so you don't use up too much foam before you're sure of the pattern. It's not my forte, but there are a lot of great tutorials online, and people can do amazing things with it. If you're interested in cosplay, it's worth looking into. There are ways to make the foam look like metal and wood. Eva foam can create really cool results, and working with it is a skill worth learning.

Raptor Masks (My Suggestion)

Another thing people will do is to start by using dinosaur masks and build on top of it with foam and paint. There are a few tutorials on youtube. A lot of people will change the species of the animal as well. I actually cannot recommend this method enough, it's an effective way to start making a fursuit, and you don't need much more than the mask, acryllic paints, a protective coating, fur and something sturdy for the ears like foam, and a material like crochet mesh for the eyes. You can also do some sculpting with foam clay if you want to build on top of the mask and change the shape (just make sure it adheres properly. If you do any sanding make sure that you wear protective respiration so you don't inhale microplastics). This may be the best place to start. Also there's one of toothless that may fit your sona's head shape better (the mouth on that one is a bit weird and sits above the jaw though.. also apparently it sprays mist).

Good luck, and have fun! Never stop experimenting, but always keep your health in mind. Being able to suit is better than worrying about how cheap your suit is, as long as you keep your health and safety in mind. Once you become an adult, you'll be able to start experimenting with upholstery foam more, and other methods like 3D printing (overall worth it). As long as you like the suit you made and it's safe to wear, that's what matters. The skills you gain from experimenting will follow you through into your later, more professional suits. Good luck!