How do you know which sound to pronounce for korean consonants?

A Bieup is pronounced like a b. A Rieul is pronounced like an l and an r mushed together. It's a bit hard to do, so I would recommend pronouncing it like an l. But when you see a romanized version of a word with a Rieul in it, you'll see it as an r (e.g. I love you=Saranghae rather than Salanghae, which I believe is closer.)