Start with the Arabic letters
First learn the 28 Arabic letters, their shapes at the start, middle and end of a word, and their sounds. Then learn the short vowel marks and the sign for a doubled letter.
Use a beginner book such as a noorani qaidah style primer, or join a class with a teacher. A teacher can hear your mistakes and correct them, which a book cannot do. Many people also learn well with a trusted online teacher.
Do not feel shy about starting as an adult, and do not lose heart if reading is hard for you. The Prophet ﷺ said that the one who recites the Quran and stumbles over it, finding it difficult, will have two rewards. Every learner begins with the same letters.