On non-graphing calculators(basic scientific calculators), you can only press:
Press "22"
Press "÷"
Press "7"
Press [ENTER]
On graphing calculators(like my TI-84{Plus CE}), you can press:
Press "22"
Press "÷"
Press "7"
Press [MATH]
Press [ENTER] (>Frac)
Press [ENTER]
And there you go!
P.S. If you want it as a decimal, here you go: 3.143