Fat
Thick
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of the fat? It's the same: fat.
ふとい
This is the kun'yomi reading of the kanji (because it's a single kanji, all alone with hiragana attached). The part you have to remember is the ふと portion. Let's use a mnemonic.
The fat guy looked down and couldn't see his foot (ふと).
山田さんのまゆげはとても太いです。
Yamada’s eyebrows are very thick.
このおみせのめんはおもったより太かったです。
This shop's noodles are thicker than I expected.
おれの母ちゃんは、ちょう太いんだぜ。
My mom is so fat.