اول ما فهمت الرسالة، رديت وقلت لها اني اقدر اساعدها.

اول ما means as soon as, once, or the moment that.

So اول ما فهمت الرسالة means something like as soon as I understood the message.

This is a very common Egyptian structure:

• أول ما وصلت، كلمته = As soon as I arrived, I called him
• أول ما شافني، ضحك = As soon as he saw me, he laughed

Because the verb itself already shows the subject.

In Arabic, you often do not need to say the subject pronoun if the verb makes it clear. Here:

• فهمت = I understood / you understood

The ending -ت can refer to different people depending on context, but in this sentence the context clearly means I.

So Arabic often works like this:

• فهمت الرسالة = I understood the message
• رديت = I replied
• قلت = I said

Saying أنا would be possible, but unnecessary here.