Learn Arabic (Modern Standard): سأكتب الرسالة حالا.

Yes, you can! Both سـ and سوف mean "will" and are used before present tense verbs. سأكتب is often used for something happening very soon (like "right away" in this sentence), while سوف أكتب can sometimes imply a slightly more distant future, but they are mostly interchangeable.

How do we know who is writing in سأكتب if there is no separate word for "I"?

The letter أ right after the future prefix سـ tells us it is "I". In Arabic, present tense verbs change their first letter to show who is doing the action. The root is كتب (to write), and adding أ at the beginning makes it أكتب (I write).

Why does حالا have an extra alif with two small dashes on top?

Those two dashes are called "tanween fatha" and they make an "an" sound at the end of the word. In Arabic, this ending is frequently used to turn a word into an adverb that describes how or when an action happens. Here, it creates the meaning of "immediately" or "right away".

Does the pronunciation of "the" change when saying الرسالة?

Yes! Because the letter ر is a "sun letter", you do not pronounce the "l" sound in the definite article ال. Instead, you skip the "l" and emphasize the ر, making it sound like "ar-risala" rather than "al-risala".