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 never leaves his futon (ふと).
太い木がたくさんあった。
There were many thick trees.
このジーンズ、少し太いかな。
Are these jeans a little thick?
わたしの毛は少ないけど、太いんです。
I don't have a lot of hair, but it's thick.