Welsh is far more phonetic than English so you can get away with a dozen or so "rules".
Some are:
"ff" = English "f";
"f" = English "v";
"ll" = Hmmm... well, make the "l" shape with your tongue and then blow - a bit like aspirating a "h" but with the mouth ready to say "l"..
"dd" = "th"
"c" is always hard as in "Car", "Catch" etc
"ae" is a diphthong = English the "i" sound in "Like"
"w" = "oo" as in "spoon"
u - "ee" as in "weed".
"r"s are trilled.
"y" = awkward.. ."i" in "cliff" if in the last syllable, otherwise as ""uh". Y on its own (= "and") is "uh" e.g Betws-Y-Coed (Betoos-uh-Koed)... Mynydd (mountain) is a good word to remember how to pronounce ("munnith") as it incorporates both "y" sounds and the "dd" = "th".
llanfairpwllgwyngyll and Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch are one and the same. The longer name was created as a tourist attraction/stunt in Victorian times... I know Llan is "church" but that's all I can translate on my own...
I'm sure one of our Welsh brethren can correct any of that that I got wrong...
Cheers, Bob.