Most of the comments here are totally correct, but perhaps may not be fully explaining the answer to the asker, which is asking for help because they are not native in English. Jalene, I am quite certain there is no term for this. It is just "the first letter of the syllable, as Leo indicates. "onset" is the closest thing ,but that really is a description of the sound, not the letter itself. As you know, one letter may have different sounds, and some sounds are made by combinations of letters.
While Tchrist is correct that a syllable has a sound, it doesn't really address the question, and syllables of course "have" letters, which are used to represent those sounds when written.
There is probably no word for this, because there is generally no need. An acronym uses the first letter of each word, generally https://www.merriam-webster.com/dictionary/acronym no syllables. This is not a common think to do, to initialize syllables, and I struggle to think of a good usage for it.