Twenty-four letters. You already know how this goes.
Greek took the same Phoenician letters Hebrew uses — alpha is alef, beta is bet, gamma is gimel, delta is dalet — then did what Hebrew never did: it promoted some of them to vowels, written on the line like any other letter. No pointing system to learn this time. Pronunciation follows the Erasmian / seminary standard used by Greek grammars and Bible colleges. (A reconstructed Koine pronunciation exists and is closer to what John's first readers said; Erasmian is what your grammar, your professor, and this site use.)
Numbers: like Hebrew, every Greek letter doubles as a numeral — that's the system behind χξϛ = 666 in Revelation 13:18. The gaps in the sequence (no 6, no 90, no 900) belong to three archaic letters that died as letters but kept their numeric jobs: ϛ stigma = 6, ϟ koppa = 90, ϡ sampi = 900. You'll meet them only in numerals.
Two kinds of trouble: Greek letters that resemble each other, and Greek letters that resemble Latin letters they have nothing to do with. The second kind is sneakier — your English-reading reflexes will fight you. Learn the tells.
Hebrew hangs its marks under the letters; Greek floats them on top. Most are recognition-only — but breathing marks change the word. Learn those cold, nod at the rest.
Every word that starts with a vowel (or ρ) carries a breathing mark. Rough ῾ means say an h first; smooth ᾿ means no h — just the vowel. That single curl is the only difference between whole words:
Nearly every Greek word carries one accent. They marked musical pitch in classical times; by the Koine era they were settling into plain stress — stress the accented syllable and move on. Recognize the three shapes; leave the placement rules to second-year grammar.
Letter data hand-compiled for this site; pronunciation follows the Erasmian convention of standard grammars (Mounce, Machen). Greek type: Gentium Plus.