how to pronounce "les" and "des" in french?
are they like "mes"？

Question

Answer

Usually, yes, but people disagree on this. Some say "mé", others say "mè". Even native French speakers are confused! :)

In the end, it doesn't really matter, because the grammar of your sentence will clear up any confusion, between "mes" and "mais", for example.

http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=3074

Answer

Yes "les" sounds like the English "lay" and "des" sounds like the English "day"... 
That is only before a noun with a consonant.  Before a vowel, you will hear the "s" and it will sound like a "z".

Don’t miss out on the opportunity to learn a language from the comfort of your own home. Browse our selection of experienced language tutors and enroll in your first lesson now!