Have Space Suit, Will Travel

A science fiction novel for young readers by American writer Robert A. Heinlein, originally serialized in The Magazine of Fantasy & Science Fiction (August, September, October 1958) and published by Scribner's in hardcover in 1958. It is the last of the Heinlein juveniles.

Plot summary:
Clifford "Kip" Russell, enters an advertising jingle writing contest, hoping to win an all-expenses-paid trip to the Moon. He instead gets a used space suit. Kip puts the suit (which he dubs "Oscar") back into working condition.
Kip reluctantly decides to return his space suit for a cash prize to help pay for college, but puts it on for one last walk. As he idly broadcasts on his shortwave radio, someone identifying herself as "Peewee" answers and requests a homing signal. He is shocked when a flying saucer lands practically on top of him. A young girl (Peewee) and an alien being (the "Mother Thing") flee from it, but all three are quickly captured and taken to the Moon.