Some words are only written in hiragana or katakana, while some words have kanji to simplify the writing (although sometimes the kanji is harder to write than the hiragana characters). When learning Japanese, I'd recommend you learn hiragana first and then kanji. Kanji is used mainly to make your sentences shorter, but one kanji can have several readings for it. A Japanese sentence never ever has all kanji. It consists of both kanji and hiragana, so I'd start with hiragana first and work your way up.
下 has a few readings such as か (ka), げ (ge), した (shita), しも (shimo), もと (moto), さがる (下がる, sagaru), ください (下さい, kudasai), おりる (下りる, oriru), etc. note: all the other characters besides 下 are hiragana.
In a sentence
(Note: Relationals are は/wa, の/no, etc.)
今日は金曜日です。Kyou wa kinyoubi desu. Today is Friday.
僕の名前は中谷ブライソンです。Boku no namae wa Nakatani Bryson desu. My name is Bryson Nakatani (boku=male form of I)
If you can read hiragana, you'd notice that although は is ha in hiragana, but in sentences it's pronounced as wa because it's a relational. This is one of the reasons why I believe learning hiragana is really important to learn first. It has its own rules, and you should learn it first because of this. If you start with kanji, you'll have to learn backwards, which can be pretty difficult.
何時ですか。Nan ji desu ka. What time is it?
三時五分です。San ji go fun desu. 3:05.
Time (minutes) - WIth hiragana, you can clearly tell the difference between 分 (pun/fun), so it's easier to know when you'd say pun/fun. Of course when you're finished with figuring out when to say it correctly, you should use the kanji, but hiragana is good for beginners in Japanese :)
1) いっぷn ippun
2) にふん nifun
3) さんぷん sanpun
4) よんぷん yonpun
5) ごふん gofun
6) ろっぷん roppun
7) ななふん nanafun
8) はっぷん happun
9) きゅうふん kyuufun
10) じゅうっぷん jyuuppun
Kanji is useful when you know how to use it and know its readings (onyomi and kunyomi), but if you don't know hiragana, you may be in a fix.