I'm a French speaker and actually I have some problems with the sounds l , r and o in lawyer. Do you have any advice for me on how to place the tongue and so on?
You might try looking at the videos and animations at Phonetics: The Sounds of American English. (I'm assuming that you are looking for General American pronunciation, not RP or other flavors of the language.)
L and R will be under Liquids (although for the R in "lawyer," you might do better to use ɚ from Central Vowels instead).
Oddly, they don't have any sound that really corresponds to the stressed vowel of "lawyer" - the closest, to my ear, is ɔ among the back vowels, although for "lawyer" and similar words the lips are rounded a little more.
When I pronounce l, my tongue either protrudes forward, touching the bottom of my upper lip, with the center of my tongue touching the bottom of my teeth; or bends up and flattens in the front of the roof of my mouth, pushing against the back of my top teeth.
When I pronounce r, my tongue is either in a similar position to the second position noted above for l, but with the tongue not quite touching the roof of my mouth; or curled back slightly in the center of the roof of my mouth.
Both consonants are voiced.
As David Wallace wrote, there is lots of variation in pronunciation. The answer to the question, depends on which form of English you learn. This link is a good one, because it has sound recordings of the different ways in which this word is pronounced in British and American English. http://dictionary.cambridge.org/dictionary/british/lawyer?q=lawyer