Class first. Marks second.
Every Thai consonant belongs to one of three classes: Mid, High, or Low. The class determines what tone a syllable produces — with and without tone marks. Most apps teach marks in isolation. That's backwards.
Class × Tone mark = Resulting tone
This table is all of Thai tone theory. Memorize the class of each consonant and you can derive any tone.
Your learning path
1. Recognize the 9 mid-class consonants
2. Recognize the 11 high-class consonants
3. Tackle the 24 low-class consonants
4. Practice reading syllables by tone
5. Read real Thai words without romanization