Actually, I'd recommend not saying much in Thai at all until you've had some practice.
The r/l problem is really the least of your worries. The real issue is tones.
Like Chinese, Thai is a tonal language. I think Chinese has four tones, but I know that Thai has five. Although most words don't actually have meanings in all five tones, they often have meaning in at least two, if not more, and this means when you open your mouth to say some simple word you are more likely to say some other word that makes no sense at all in that context and that will earn you a puzzled look at best and, at worst, further confirmation that farang - foreigners - are a bit stupid and can't speak Thai or eat rice. And we don't really need more confirmation of that, now, do we?
Incidentally, tones make the Thai language fertile ground for puns and wordplays. Sometimes I make hilarious jokes without even meaning to, just by mispronouncing something.
When I was learning Thai my teacher had me chant the 42 consonants of the alphabet, plus the sound "ah", in all five tones, just to get me used to saying them. In spite of all this dull practice, it took me about a year to be able to reliably hear the tones, and another year to be able to reproduce them with any consistency and accuracy.
If you're tone deaf, it may never work.
Besides that alarmingly large number of consonants, there are also about 18 vowels which can be combined to form a whole lot more sounds. Plus four tone markers. So learning to read Thai can be even more confusing than learning to speak it for some. However, I'm a literary learner, so for me being able to read and write simple Thai helped me to learn.
Notice that I said simple Thai. Thai is an old and complex language which has a grammatical structure very different from European languages. When I look up many words in my Thai dictionary, it says they are nouns, verbs, adjectives, and adverbs, all without any changes in form. Yikes! Compounding the problem of comprehension is the fact that Thai doesn't have sentences, but rather clumps of words which together, make a thought. The clump contains no spaces, so it's quite difficult for the neophyte to tell where one word ends and the next begins, let alone what each word means separately and in the context of this particular clump. And it's precisely that context which determines whether a word is a noun, verb, adjective, or adverb in any one instance. Double yikes!!
Needless to say, even after four years of study, my Thai reading remains rudimentary, though I can speak quite well.
You can see what Thai writing looks like, and get some interesting information on the Thai alphabet and the problems of translating Thai here in everything at Thai, or outside at:
seasrc.th.net/main/main.htm | 
