The pronunciation of French words can be tricky.
Indeed, with an incorrect pronunciation your sentences would sound very weird.
For instance: J’ai acheté une robe jaune. If you don’t pronounce the [au] of “jaune” like an [o] sound and pronounce it like “jeune”, you would then be saying “I bought a young dress” instead of “I bought a yellow dress”!!
Here is a list of 14 words which you will want to pronounce well:
jeune = young ≠ jaune = yellow
cuisine = kitchen ≠ cousine = cousin girl
cheveux = hair ≠ chevaux = horses
cousin = cousin boy ≠ coussin = cushion
soeur = sister ≠ souris = mouse
dessert = dessert ≠ désert = desert
poisson = fish ≠ poison = poison
To help you, here is a recording of the pronunciation of these words.