You can find some general rules here... http://www.abcfastphonics.com/vowel-rules.html The good news is that our 5 vowels (sometimes y too), which show up in almost every word, typically have 3 sounds each, the short, long and schwa sound. So learn the 15 sounds and pronunciations become much easier.

IN general (most cases) a vowel followed by two consonants will have the long sound (apple) while a vowel followed by a single consonant and then a vowel will have the short sound (date). Senate and senator are exceptions to this rule. Unfortunately, every language has exceptions! To sum up: vowel - consonant - consonant -> long sound vowel - consonant - vowel (usually an e) -> short sound

There are no reliable rules. Us native speakers probably learn them by discovering groups of words that have similar spelling and similar pronunciation--we see "spate" or "abate" or "sate" and unconsciously think "the -ate is probably the same as 'date' or 'fate' or 'hate.'" And then we see "separate" and we just know which word it is and how it's pronounced--"separət" when it's an adjective--and we keep it in a separate compartment. It helps to familiarize yourself with the clusters, it helps to remember "this word isn't pronounced the way you'd expect," but it probably is not worthwhile to try to formulate or memorize rules--which are always creative inventions and always have many exceptions.

The others have answered... I'll just add... DON'T worry too much. There are lots of exceptions to the rules and it can be confusing. It's even worse with English spelling! Oh! All those WEIRD spellings! Be patient, listen to a LOT of native speakers (TV/cinema/radio) and GRADUALLY the sounds will become familiar. I'm trying to learn Hindi and Tamil, and believe me, I keep having to tell myself to be patient and not panic!!!!! I hope MY advice to MYSELF will help YOU remember to go slow, and not worry!