I am going to travel to japan the next spring and I just want to be able to read most of the signs (and of course speak some Japanese). Any answers on what is the most common to use on signs: Katakana, Hiragana or Kanji?
Statistically speaking, the answer is definitely kanji, because the vast majority of Japanese nouns (including place names) are written in kanji. For example, on signboards, Tokyo is 東京 (kanji), Ginza is 銀座 (kanji), subway is 地下鉄 (kanji). However, there are over 1000 common kanji each with more than one reading, while there are only 40-some hiragana/katakana. It usually makes no sense to remember only kanji, but if all you want to do is to identify a few words for fun, you may choose to do so. By the way, almost all important signs have romaji like this, too.
I recommend you read a basic article about the Japanese writing system. Japanese uses a combination of kanji, katakana and hiragana to write a sentence. This means a typical Japanese sentence has all kanji, hiragana and katakana mixed together (see example here).
The road signs in most places in Japan have been standardised, such that any directional signs will typically have both Japanese (typically kanji with kana where appropriate) and English for place names. Most other signs will either have just a symbol or include a small amount of Japanese (e.g. 止まれ), possibly with English as well.
As for shop signs, they can vary widely. As you can see in the images in this Lonely Planet article, there are some recognisable logos, some bilingual signs, and some signs completely in one language.
If you don't know much Japanese yet, then reading signs will be a combination of luck, pattern recognition, and having a good reference to work from. Getting your hands on an English or bilingual map of the area you're going to be in will be extremely helpful, as will having a mobile internet connection (for maps and also possibly automated translation through Google Translate or a similar service). Many Japanese people speak some level of English, so you may survive just by looking sufficiently helpless and asking for directions.