You will need:
A ballon (round balloons irk but the long ones work best)
Fishing wire or kite string
A plastic straw
Firstly tie your string to one thing put the other end through your straw and then to another thing (a chair to a door knob) Make sure your string i yied nice and tight and sort of straight. Then blow up your balloon put pinch the end instead of tying it and tape it to your straw. And watch your rocket go!