Learn Hardcore German: Der Kaffee hat heute eine starke Wirkung, sodass ich schnell wach bin. - The coffee has a strong effect today, so that I am quickly awake.

Result: Der Kaffee hat heute eine starke Wirkung, sodass ich schnell wach bin. (Because of the strong effect, I end up awake.)
Purpose: Ich trinke Kaffee, damit ich wach werde. (I drink coffee in order to become awake.)

Is "sodass" one word or two? I’ve seen "so dass" as well.

Both sodass and so dass are correct. The single word sodass is the standard recommended spelling.
Be careful not to confuse this with the correlative construction so … dass (two separate words far apart), used with an intensified adjective/adverb:

Er ist so müde, dass er einschläft. (so + adjective + dass)

Do I need the comma before "sodass"?

Yes. Sodass introduces a subordinate clause, and German requires a comma before subordinate clauses:
…, sodass ich schnell wach bin.

Why is the verb at the end in "sodass ich schnell wach bin"?

Because sodass starts a subordinate clause, which in German has the finite verb at the end:

Main clause: Ich bin schnell wach.
Subordinate clause: …, sodass ich schnell wach bin.

Could I say "sodass ich schnell wach werde" instead of "bin"?

Yes.

… wach bin emphasizes the resulting state (I end up being awake).
… wach werde emphasizes the process of becoming awake.
Both are idiomatic; choose based on focus.

Why is it "eine starke Wirkung" and not "einen starken Wirkung"?

Wirkung is feminine.
It’s the direct object of hat, so it’s in the accusative.
Feminine nominative and accusative both take eine and the adjective ending -e: eine starke Wirkung.
"Einen starken Wirkung" would be wrong because that mixes masculine endings with a feminine noun.

Why "Der Kaffee" (nominative with "der") and not "den/dem Kaffee"?

Der Kaffee is the subject (the thing that “has” the effect), so it’s nominative. Der is the nominative masculine singular article. Den is accusative; dem is dative—neither fits here.

Could I say "Der Kaffee wirkt heute stark" instead of "hat … eine starke Wirkung"?

Yes. Wirken (“to have an effect”) is a natural verb alternative:

Der Kaffee wirkt heute stark, …
The meaning is essentially the same; the original uses a noun phrase (hat eine starke Wirkung). Both are idiomatic.

Where can "heute" go in the sentence?

Common options:

Heute hat der Kaffee eine starke Wirkung, … (fronted time = emphasis; triggers inversion)
Der Kaffee hat heute eine starke Wirkung, … (neutral)
Der Kaffee hat eine starke Wirkung heute, … (possible but less neutral; often used for end-focus)

Is "schnell wach" the right order? Could I say "wach schnell"?

Use schnell wach, not wach schnell. The adverb schnell normally precedes the predicative adjective wach:

Ich bin schnell wach.
In the subordinate clause: …, sodass ich schnell wach bin.

Should I add "auf mich" after "Wirkung" to say the effect is on me?

Optional.

General statement: Der Kaffee hat heute eine starke Wirkung, …
Explicit target: Der Kaffee hat heute eine starke Wirkung auf mich, …
If context already makes it clear the effect is on you, you can omit auf mich.

Can I just use "dass" instead of "sodass"?

Not for a consequence clause unless you have a preceding intensifier like so/dermaßen/derart within the first clause.

Consequence (as written): …, sodass ich …
Intensified correlative: Der Kaffee hat heute so eine starke Wirkung, dass ich …
Using plain …, dass ich … after the original main clause would lose the result meaning.

Could I replace "sodass" with "deshalb/darum/daher"?

Yes, but then you have two main clauses and inversion in the second:

Der Kaffee hat heute eine starke Wirkung; deshalb bin ich schnell wach.
(Also works with darum or daher.)

Why present tense "hat" and not past ("hatte") or perfect ("hat … gehabt")?

German often uses the present for current states/effects, even if they started earlier today.

hat = it has (now).
hatte = had (then, no longer true).
hat … gehabt would sound like a completed effect earlier today; it doesn’t fit if the effect is current.

Why is "Wirkung" capitalized but "wach" isn’t?

All nouns are capitalized in German, so Wirkung (a noun) is capitalized. wach is an adjective (used predicatively here) and stays lowercase.