Many dictionaries such as Hayyim
's give transliteration. The best online English-Persian dictionary, in my opinion, is Farsi123
, which doesn't give transliteration, but you can always copy words from there (or anywhere) into Hayyim's dictionary and get the transliteration. I would also recommend you buy Yavar Dehghani's Persian-English/English-Persian Learner's Dictionary which lists all words with Persian spelling, Roman transliteration, and English translation. The nice thing is that the Persian-English section is organized by transliteration, so you can look up Persian words without knowing how to spell them. I think it's one of the most useful dictionaries for total beginners.
Please correct my mistakes in any language.