I want to improve my salah but I do not know where to even begin, there is so much I am doing wrong. What is the one thing I should fix first without drowning in guilt?

Let me take the weight out of that word before we speak of what to fix. There is no drowning here, and no need for guilt, because the One you stand before never asked for everything at once.

“Allah does not burden a soul beyond its capacity.”

Qur’an, Al-Baqarah 2:286 · meaning

Read that slowly and let it settle. If your Lord Himself refuses to overload you, then the pressure to repair everything today is not coming from Him. It is the old habit of an anxious heart, and you can set it down. A small step taken with love pleases Him far more than a grand plan that collapses under its own weight.

The Prophet, peace be upon him, answered both of your difficulties in a single breath, the not knowing where to begin and the guilt over all that still feels unmended:

“The religion is easy, and no one makes the religion hard upon himself except that it overcomes him. So aim true, draw near, and receive glad tidings; and seek help through worship in the morning, in the late afternoon, and in a portion of the night.”

The Prophet (peace be upon him), Sahih al-Bukhari (39) · meaning

Notice what he did not say. He did not say the religion is heavy and you are failing it. He said the weight comes from the man who piles it on himself, and wanting to mend the whole of your salah in one day is exactly that piling. Then he showed the way through: aim true, draw near, and receive the glad tidings, for a heart that is trying is promised good news, not condemnation. And take your worship in portions, at the hours you can actually keep. The same mercy speaks in:

“So recite of it what is easy for you.”

Qur’an, Al-Muzzammil 73:20 · meaning

So here is the one thing, and only this one for now: presence in the Fatiha. Not the whole prayer, not every posture, not years of habit reformed in a week. Only that when you recite the opening of the Book, your heart is awake to the words. A change you can keep is worth more than one you cannot, and this is small enough to keep for a lifetime. Let the rest come later, in its own time, the way dawn arrives light by light rather than all at once. Fix this one thing, and you will find the rest of your salah quietly following the heart you have woken.

One small step: In your very next salah, give your full attention to the Fatiha alone. If your mind wanders elsewhere in the prayer, do not scold yourself; simply return to presence when the Fatiha comes again.