Watashi is commonly used version.
Watakushi is more polite version. It's for formal situation(about a speech, when speaking to your customer).
And more
http://en.wikipedia.org/wiki/Japanese_pronouns
And, Watakushi and Watashi are only I. not I am.
"I am" is watashi -desu.
Roughly speaking, wa makes a previous word a topic or the subject.
Who is Naruto? -- I am.
Naruto wa dare? -- Watashi desu.
Who are you? -- I'm Naruto.
Anata wa dare? -- Watashi wa Naruto desu.
For your help.
