The conventional exclamation for disgust is yuck (alternatively, yech); the Macmillan entry says it is
used for saying that you think someone or something is dirty, ugly, or unpleasant
Oh, yuck! It’s slimy!
The earliest OED entry is from 1966, noting an earlier verb sense of yuck meaning to vomit, but it may have existed earlier in dialectic English.
Chris Sunami's suggestion of ew is also a good one.
Somewhat milder is ick, a reaction to something icky. Somewhat stronger is blecch (blech, bleech, etc.), which has not made it into any of the major dictionaries, but is easily found online, and even featured in the title of an episode of The Simpsons. Stronger still is bleah, also imitative of retching or gagging, though OLD labels this as North American.
Other onomatopoeia for vomit or vomiting, like barf or yack or hork may be suitable, but the degree to which these terms is understood will vary greatly by region and generation, and some slang like woof or hurl may thus be ambiguous.
A broader term for something generally unpleasant is ugh, as noted elsewhere, but this is more a term for dislike or disappointment.