Let's do some simple operations.  Place your cursor in each of the equations below and click shift-enter to see the result.  Check the result in your head or with a calculator.
 First steps:   place the cursor on the cell (near statement, number or equation) . Click SHIFT -and-ENTER together
..... the line below is linked to the first cell, it is the answer line

Basic math Assistant Pallete (Palletes, pool down menue) or conventional notation

a*(a^2)

a^2

2+2

e

(2^(1/2))

a=2

(a^(1/3))

a=2.

(a^(1/3))

(a) ^(1/3)

Clear[a]

You can use Basic Math Assistant Palette.    
Basic math Assistant Pallete (Palletes, pool down menue)

Try these:

 1.  the 7th root of 1000,   
 2.  5!, 
 3.  (e^2)

(1000^(1/7))

(1000.^(1/7))

N[(1000^(1/7))]

5!

(E^2)

N[(E^2)]

Try now Mathematica’s built in functions (Sin, Cos, etc.) .  If you do not know their syntax go to  Help -> Documentation Center or     ->Function Navigator

Notice: built in functions have firtst letter in  upper case and have square brackets for variables

Clear[x]

ArcSin[x]

x=0.5

x

ArcSin[x]

Sqrt[21]

Continuue and check the values of functions in different basis or representation/units
1.  ln  of 2.718
2.  log of 100
3. cosine of π/4

you can use Pi for π

Log[2.718]

Log[10,100]

Cos[π/4]

Make your own functions!!!

myfunc[x_]:=((x )^2)

and apply

myfunc[4]

Plot  (draw) the function y = (x^2) from x = -4 to x = +16.

Plot[myfunc[x],{x,-4,4},AxesLabel->{"x","y"}]

mysin[x_]:=Sin[x]

Plot[mysin[x],{x,-16,16},AxesLabel->{"x","y"}]

Plot sin(x) and sin(x+ϕ), with ϕ the phaseshift  factor

ϕ=π/2;

Plot[{Sin[x],Sin[x+ϕ]},{x,0,10π}]

Try Plot Range  and other graphics options

Try two variables function

.....

Try Solve

Try Derivative