Thai consonant order is as in the following table. Read the first block from left to right, top to bottom (i.e. ก to ง, then จ to ญ &c.). Then read the columns f, g and h from top to bottom. (The logic of this arrangement is explained in Notes On ... Consonant Order.)
ฤ (whilst not normally considered a consonant) is treated as a consonant and is ordered between ร and ล.
You can practice putting groups of consonants into the correct alphabetic order by clicking on the “Try It” button below.
If two words start with the same character, then order them by the next character.
If one word starts with a shorter word, then the shorter word is ordered first. For example, ทบ is ordered before ทบทวน and ทบวง.
อ is always treated as a consonant, even if it's functioning as a vowel.