which words in japanese use kanji?

Foreign words and sounds are usually in katakana. Everything else is a combination of hiragana/kanji.

Foreign word & sound examples: menu, coffee, sounds that animals make.

Katakana can also be used to stress something, much like italics in English. So when you want to emphasize a word, write it in katakana. Also, sometimes local dialects are written in katakana, which gives them a foreign feeling since it's not standard Japanese.

Hiragana is often the words that string together things in a sentence, such as "in a" in the words I just wrote. "things" and "sentence" would've been in kanji.

Most words are tangible objects, places, or concepts. There's usually kanji for it. It used to be when Japan was first adopting foreign words and place names, they would use kanji for it. But gradually the word changed to a foreign-adopted word (rather than making a Japanese word up for a foreign thing) and being written in katakana. Baseball (yakyu) is still said the Japanese way and in kanji but people know what beesubooru is (that's the way Japanese would pronounce it and write it in katakana). America (country) used to be in kanji but now just katakana. Camera used to be shashinki in kanji (picture machine) but is now simply "kamera" in katakana.

The word for walk would be in kanji, but the verb tenses would be in hiragana, such as in English it would be walk-ed, walk-ing, etc.

Only a few words I can think of off hand are in hiragana/katakana only: ramen, frog, thank you, etc. Ramen and frog (kaeru) are curious examples because they are Japanese but for some reason, tend to be written in katakana, which is reserved for foreign words. They can be written in hiragana and they are sometimes, just look at businesses and books, but these words are very commonly written in katakana. It's possible there might be some historical leftover reasons for this. At various times in Japanese history, katakana was the standard way of writing, not hiragana, or that women used one and men used the other. For example, pre-WWII katakana was standard.

In one Japanese sentence there can be a whole bunch of kanji.

Such as: (I'm on my phone but I know these have kanji).

Gogo ichiji ni, onna no hito wa ie ni kimashita. Kanojo wa kirei de, yasashikute, kawaikute, ureshii hito deshita= At 1 pm, a lady came to my house. She was a pretty, gentle, cute, and happy person.

Most particles have no kanji, most words in Japanese do have kanji, only foreign words are in Katakana.