Regrettably, there are no reliable rules for spelling or pronunciation in English. Pronouncing the double "ee" as /i:/ is fairly reliable, but there are exceptions, such as "preempt" (two syllables) and "deemphasize." What do native speakers do? We learn some of the more common patterns in school, such as the effect of a silent "e" at the end of a word--as illustrated in this Tom Lehrer song: https://www.youtube.com/watch?v=91BQqdNOUxs When we see a new word that has a similar pattern to one we know, we guess that there's a similar pronunciation. Very often, we guess wrong. Whenever it's important, we check a dictionary. We really do. We have to. The spelling of a word is, at best, a useful reminder of pronunciation. There has long been a debate about the best way to teach English speaking children how to read. The debate is referred to as "phonics versus sight-reading." The short answer is that most teachers use a combination of methods. "Sight-reading" means to recognize a whole word as a unit, and learn the pronunciation of the word as a unit--almost as if the word were a ideogram. English spelling and pronunciation present a challenge that is not found in phonetically-spelled languages like Portuguese and Spanish. Unfortunately, that is just the way English is, and probably isn't going to change any time soon. My mother tells a story of being humiliated when she gave a talk at school about "atoms" and "molecules," words she had only seen in print, and pronounced the words as "AY-tums" and "MOLE-cyools" (instead of "MOL-uh-cyools.")

If you are studying by yourself, look for phonics books for children. The phonics books teach children the common spelling to sound relationships. If you are looking for lessons, ask the teacher how they teach pronunciation. A good answer will include teaching how to make the sound (articulatory phonetics) and how to pronounce words based on their spelling (phonics).