Japanese is considered a difficult language to learn. This is certainly true for native speakers of European languages, such as English, because Japanese is fundamentally different from European languages.
One of the biggest difficulties of the Japanese language is its complex writing system. Unless you are already familiar with Chinese characters (kanji), many years of study are necessary to achieve complete literacy. Japanese students learn about 2000 kanji until the end of junior high school and continue to learn more until the end of their school careers. The two syllabaries Hiragana and Katakana (together about 100 signs), however, can be memorized within a short period of time.
Another difficulty of the Japanese language is the fact that a person's speech can vary depending on the situation and the person. A student of Japanese has to become familiar with Japanese society and customs in order to understand the detailed rules of the different levels of speech.
Compared to many European languages, basic Japanese grammar is relatively simple. Complicating factors, such as gender articles and distinctions between plural and singular, are almost completely absent. Conjugation rules for verbs and adjectives are almost entirely free of exceptions. Nouns are not declinated at all, but always appear in the same form. These facts make the language relatively easy for starting students.
Note that there are more information pages in this guide about Japanese Language Schools and studying in Japan in general.
