If you're studying Japanese, one of the first sentences you learned was most likely わたしは（your name）です, translated roughly as 'My name is ・・・' (more literally: 'As for the things concerning myself, I'm ・・・'). How very important this sentence is you probably won't realize until further studies on the language.
Firstly, there is no clearer example of the core Japanese sentence pattern. subject + は + predicate + です. You might not even realize that, but with that sentence alone you really can express most of what you want to say, whether it's talking about yourself, somebody else, an object, an animal, literally anything else, and using either personal names, nouns and adjectives.
Secondly - it contains は particle, also called a subject particle. Later in your study the problem that will weigh on your mind the most will probably be the difference between は and が, a so-called subject particle. The easier way one could put it is: generally, they mean more of less the same thing, but the latter puts more pressure on the subject, whereas the other is marking the topic with a rather vague: as for ~, concerning ~, regarding~.
And then thirdly - it contains わたし, the pronoun expressing the idea of 'I'.
And everything would be fine, people would live happily ever after... if only not that it is THE Japanese, the language which is probably the most brilliant mixture of the wonderfully painless with the totally incomprehensible.
In Japanese, they are many ways to cover the concept of 'I'. Each carries different meanings and highlights different aspects that you want to accentuate about yourself.
For example, わたし, watashi, is the most safe pronoun you can use as a beginner. It's polite and can be used by both sexes. Alas, in informal conversation it is considered slightly feminine, and it resembles another ultra-feminine pronoun, あたし, atashi, so in order not to be perceived as too effeminate, boys quickly learn to use 僕, boku, or 俺, ore. While 僕 is fairly neutral, 俺 sounds really strong if not arrogant, and highlights the feeling of superiority towards the person one is talking to. If you're talking to a 目上, me-ue, person higher in status than you, and you want to show your humbleness, you use わたくし, but in a club of interest or a team of people, one tends to use 自分, jibun, instead. Amidst the eldery, also わし can be heard.
(Accordingly, there are numerous pronouns expressing the concept of 'you' (like あなた, anata, 君, kimi, おまえ, omae), and the valuating titles you add to the name of the person you’re talking to (like ~さん, ~ちゃん, ~君（くん）, ~様） but maybe I'll plunge into it some other time.)
All of that is even more confusing when one realizes that the practice sometimes doesn't necessarily follow the theory. For instance, young high school girls tend to use 俺 while talking to themselves A LOT. Increasingly more and more women are using 僕 as a form of statement against sex divisions in the language. While talking about one's feelings/ opinions/ fears/ concerns, 自分 is considered the most appropriate, as it is not too intrusive concerning the privacy of the other.
And the most funny thing is... that in daily conversation, when it is obvious that the person is talking about themselves, 'I' pronouns are generally omitted.
There is the perfect phrase in Polish that summarizes it all: I bądź tu człowieku mądry! :-) (Roughly translates as: And be smart [in all of that]!)
Here is a funny manga showing how Beef Jerky <sic!>, an American 留学生, struggles with Japanese pronouns. Enjoy!
(for those who didn't get the joke with the name - especially vegetarians and women, because my experience shows it is highly unlikely not to know Beef Jerky if you are a man - please, google the name)
