Let me tell you, Felipe, in English, almost all unstressed vowels are reduced to schwa, and final Ts are usually much weaker than in Portuguese. BTW, the schwa is the most common sound in the English language -- it sounds like the unstressed final "a" in Brazilian Portuguese.

That is just slang. It means: Let me tell you. Neither "lemme" or "ya" are real English words. The speaker is running the words together to sound more informal or cool. Hope that helps. :)

It's a slurry form of "let me tell you", ie. "I want to tell you something" or "I'm going to tell you something that I think is important". As with any other slang or colloquial form, don't use it in your standard English. Wait until you hear the other speaker say it (or use other slangy forms) before you decide to use it or not.