French gives you more of Thai than English does. Your p, t and k carry no puff of air, which is exactly what Thai ป ต ก need, and your b and d are voiced all the way through, as บ and ด are. Anglophones spend months on that half. You start with it.
What is new is the other half. Thai has a second set of stops, พ ท ค, with the burst of English 'pie' and 'two'. French never contrasts the two, so your ear files both under one letter until it is trained not to.
- ปาpaː
to throw
your p in 'papa'
- พาpʰaː
to lead someone
add the puff: a sheet of paper in front of your lips moves
- ไก่kàj
chicken
your k in 'cas'
- ไข่kʰàj
egg
the same, with the burst
Vowels you already own
เ is the é of 'été' and แ is the è of 'père'. โ is 'eau' and ออ is the o of 'or'. These four are hard for English speakers, who slide them into diphthongs. For you they are free.
The one vowel French does get wrong is อึ: say 'ou' and spread your lips into a smile without moving your tongue. French rounds it back into 'ou' by reflex, so check in a mirror.
Three French habits to switch off
Nasal vowels. French turns -an and -ang into a nasal vowel and drops the consonant. Thai keeps the vowel oral and pronounces the n or ng in full, and คน, คม and คง are three different words.
Released finals. French releases a final consonant, and often adds a small vowel after it. Thai closes the mouth and stops: no burst, no vowel.
Phrase stress. French lifts and lengthens the last syllable of every phrase. In Thai that syllable is often the politeness particle, and lifting it turns one particle into another. The article on tone drift goes into this one in detail.
- คนkʰon
person
a real n at the end
- คงkʰoŋ
probably
a real ng, not a nasal vowel
- กับkàp
with
close the lips and stop
ร
Your throat r has no place in Thai. Use a single tap of the tongue, as in Spanish 'pero', or simply an l: casual Bangkok speech says l for ร all the time, and nobody will hear it as a mistake.