I've signed up for the challenge!

I am serious about starting to speak Russian now!

Я всерьез решила начать говорить по-русски!

Я серьезно начинают говорить на русском! (it's not correct to use "говорить на русском". Use "по-русски", "по-английски", "по-немецки" and etc)

I've signed up for the challenge!

I am serious about starting to speak Russian now!
Я серьезно начинаю говорить на русском!

I am serious about starting to speak Russian now!
Я серьезно начинают говорить на русском!

1) Let me try and explain this to you. You have a problem in the "am serious about" part.
In English we have a "verb + adjective + preposition + verbal noun" construction (am + serious + about + starting).
But in Russian you said "Я серьезно начинаю", which, if translated into English, would become this: "I am seriously starting" - see the difference?

And you don't want to say "I am seriously starting", you want to say "I am seriously thinking/concider about starting to speak Russian".

The problem is that in English we have a verb "to be", which is placed before an adjective in your sentence and it is correct.
But here - "Я серьезно начинаю" you need a verb linked to the adverb "серьезно". So, you can say it like this:
- Я серьезно думаю начать говорить на русском.

- Я серьезно задумываюсь о том, чтобы начать говорить на русском.

So, just remember, that the phrase "to be serious about doing something" is actually translated into Russian like "to seriously consider doing something".

PS: if (instead of a verbal noun) there's a simple noun after "to be serious about", you usually translate this as "я серьезно отношусь к (чему-то)".

For example, "I am serious about our relationship" - "Я серьезно отношусь к нашим отношениям"; "I am serious about my studies" - "Я серьезно отношусь к моим занятиям".

I've signed up for the challenge!

I am serious about starting to speak Russian now!
Я серьезно начинаю говорить на русском!

А лучше будет сказать - Я планирую сосредоточиться на  занятиях русским языком.

Видео на ютуб мне очень понравилось, оставила комментарий. Вы - молодец!