You can visualize complex numbers on the plane:
Graphics[Point[ReIm[{I, 2 - I, I^2, 0}]], Frame -> True]
with Mathematica 10.1 syntax, or
Graphics[Point[Map[{Re[#], Im[#]} &, {I, 2 - I, I^2, 0}]], Frame -> True]
with older syntax.
In Mathematica, the imaginary i is I (capital i), or esc+i+i+esc, or
Complex[0,1]
An example:
In[12]:= Sin[2. + 3. I] Out[12]= 9.1545 - 4.16891 I
or:
In[14]:= Sin[Complex[2., 3]] Out[14]= 9.1545 - 4.16891 I
