Unfortunately, there isn't a rule in examples like these and many others; that's partly why English is so hard for non-natives to learn. You just have to learn the words and remember how they're similar in spelling but different in sound. A major reason why modern English is like this is because it's been influenced from many languages over time, and didn't only evolve from Old English (which included the letters þ and ð).

I suggest you don't learn phonetic system. You need to find a guide to english pronunciation. Once you are used to it, you will know how to pronounce letter based on the situation. I could find a site for you, but you can type "english pronunciation guide" into google as well as I can.

There are 26 letters in English but there are 44 sounds. So spelling is not a good guide to pronunciation. What you can do is to learn Phonetic Symbols and they are really easy to learn. Each symbol has a special sound. Once you learn them you will be on your way to correct pronunciation. By the way there is no rule of Pronunciation , it is just like that you have to adopt it the way it is. You can visit http://www.bbc.co.uk/worldservice/learningenglish/grammar/pron/ to learn Phonetic Symbols and all about Pronunciation. www.atozofenglish.com