Most Korean consonants come in three versions, namely unaspirated (without a puff of air), aspirated (with a puff of air) and tensed (stressed). Unaspirated consonants exist in English too, but never alone: compare the sound of 'p' in "pot" (aspirated) and "spot" (unaspirated). Many English speakers find it helpful to pronounce an imperceptible little "m" in front to 'stop' the puff. Tensing isn't really found in English, but pronouncing the consonant quick and hard is a reasonable substitute.
b (p) ㅂ like 'p' in "spit" (unaspirated)
p (p'/ph) ㅍ like 'p' in "pig" (aspirated)
pp ㅃ tensed 'p'
m ㅁ like 'm' in "mother"
d (t) ㄷlike 't' in "stab"
t (t'/ph) ㅌ like 't' in "top"
tt ㄸ tensed 't'
n ㄴ like 'n' in "nice"
j (ch) ㅈ like 'g' in "gin"
jj ㅉ tensed 'j'
ch (ch') ㅊ like 'ch' in "chin"
g (k) ㄱ like 'k' in "skate" (unaspirated)
k (k'/kh) ㅋ like 'c' in "cat" (aspirated)
kk ㄲ tensed 'k'
ng o like 'ng' in "sing"
s ㅅ like 'ss' in "hiss", but si is pronounced like 'shi' in 'ship'
ss ㅆ tensed 's'
l ㄹ somewhere between 'l', 'r' and 'n'
h ㅎ like 'h' in "help"
While the rules above are usually correct for the first consonant, those in the middle of a word are usually (but not always) voiced, which means that ㅂㄷㅈㄱ turn into English "b", "d", "j" and "k". The best rule of thumb is to concentrate on remembering that the first consonant is "special" and the rest are more or less as in English: bibimbap (비빔밥) is pronounced "pee-bim-bap", not "bee-bim-bap" or "phee-bim-bap".
The aspirated spellings with "h" are used only in the official North Korean orthography.