A surprising number of native Britons have difficulty in spelling this word. Here it is, correctly spelled: beautiful. Learn it; remember it. Beautiful has the main stress on the first syllable and is pronounced 'byoo-tiful', IPA: /ˈbjuː tɪ(or ə) fʊ(or ə)l/. The adjectival suffix follows the normal spelling rule for -ful.
(It may help some to know that it is derived from the French word beau, which means 'beautiful', and is pronounced with roughly the same vowel sound as English 'know', /əʊ/.)