They’re not dumb, but they’re many, so I hope you don’t mind that I answer as directly as possible 
- Hard R: When R is the first letter of a word or when you have a double R (rr).
Uns carros (Some cars)
O rato (The mouse)
A rua (The street)
- Soft R: When R is the last letter of a word or when you have a single R between other letters.
Números (Numbers)
Um livro (A book)
Congelador (Freezer)
- Sibilant as in soup: At the start of a word or when you have a double S (ss).
Um sapato (A shoe)
A massa (The pasta)
- SH as in shoe: Before a consonant or at the end of a word, when not followed by a vowel or H.
Escreve (Write)
Somos (We are)
- Z as in zero: Before a vowel or at the end of a word when followed by a vowel or H.
A casa (The house)
Quantos anos tens? (How old are you?)
Joel and Rui did a good job describing all the variations of X in here 
:
How to Pronounce the Letter X in Portuguese
The double S and the Ç both sound like the sibilated S I described above. But Z always sounds like a proper Z.
The accents only affect the vowel they lie on. They also indicate the stressed syllable of the word. The pronunciation of the surrounding letters isn’t affected, though. Here, S sounds like a Z only because it’s between vowels, and it would still sound the same without the accent (for example, in the word “empresa” - company).
Surprisingly, there’s a logic to these! This should be celebrated, because there are other questions where I feel really bad when it’s just not possible to give a clear answer.