It depends how accurate/pretentious you want to be. Tofu is a really well-known "Japanese" word so in English it has taken on the phonetic characteristics of our language. It's "to" as in "toe", and "foo" instead of a Japanese "u" sound. If you want to get really technical, the word is originally Chinese so you could start pronouncing it "dòufǔ" if you want. Overall, given how well-known it is, I would probably just go with the standard English-language pronunciation: "toefoo"