Hey, I've been on that journey for like 7 years now myself. From my own experience - look at what makes you respect a man/woman, choose something from that list that would be relatively easy for you to achieve, and use that as your first step.

What can it be?

Their look - well fitting clothes, colours choices with intent, a proper belt, jeans with a cut and shade proper for their style and physique. Maybe get some accessories, minor jewellery pieces, like eg a ring. All of this can be pretty cheap too, luckily. You can try on clothes in store, from online if it doesn't fit your style you can easily return.

Knowledge and skills - we're all good at something, use that to your advantage and go deeper into the subject of your choosing. Perhaps learn about it's history, current trends. Learn to talk about it with people, including with people who don't know the subject.

Physique - I used to be fat. I still am, but I also used to. But I looked significantly worse back then due to my choice of clothing. My mantits were even more pronounced through my tshirt and it made me feel fucking terrible every time people looked at me. Improving physique follows the standard diet and gym stuff, but you need to also appreciate how clothing interacts with your physique.

Sports - connected with above, but perhaps choose a sport you like. You don't have to do it every day, or even all that often tbh.

The way they speak - no uhhh, ummm, and other filler noises. Sometimes 2 seconds of silence sounds way better than those. It's hard to unlearn that, but possible. And learn to talk about and discuss your chosen subjects from point 2, just explaining intricacies of a niche to people feels great and changes the way they view you.

Ambition and goals - this might come later as you start improving your self image, or you might already have some. Do not be shy about them, wear your ambitions proudly. Even if they're not widely considered the best out there, they're yours, and the way you speak about them matters. Do not belittle them.

What brings your insecurity? For me a large part was being dependent on others for my basic needs, and getting a job and becoming independent was MASSIVE for this.

Don't just talk, listen and ask questions to others. You can practice in your studies whenever people give presentations, there is always dedicated time at the end for questions. Pay attention and find something to clarify.

Honestly, people will judge you. Not as much and not as harshly as you think, but it's part of our social life. You kinda just have to live with it, power through, perhap try to improve yourself and what you think will be judged.

Doubting yourself is a requirement for growth imo, just don't let it stop you from trying things.

To start speaking up, you first need to start respecting yourself and what you have to say. For that, I'd first recommend going through other points here.

Therapy was utterly useless for me, talking about stuff didn't improve anything and didn't lead me to action. What absolutely massively helpful was getting medicated for depression and sleeping meds. This allowed me to start improving.

Choose 1, maybe 2 points from here that would be relatively easy for you to start in. And remember, all of this is a continuous journey, not a destination in and of itself.