...Okay, I tried to write up a simple explanation, and failed miserably. It depends on how comfortable you are with terms like "vowel", "consonant", "syllable", etc (not to insult your intelligence but they aren't really household words). And it's complicated by the distinction between simple and compound words. The only thing easy is that "w" is always pronounced "v" - Wagen, Wasser. And "v" in the beginning of simple words is "f" - Vogel, Verlag, - and "v" everywhere else (although I struggle to come up with examples for that; can only suggest "Adjektiven", a plural form, which in the singular would end with the "f" sound). Although, again, you have to look if it's a simple or compound (more often than not) word... or actually a recent borrowing, in which case all bets are off.

And compound words can come in an obvious form like Fernsehen, or innocuous like Beispiel.