If you know nothing about horses, you are making a great choice by gelding him. The stallions that people say are "sooo sweet", were usually trained by someone who knew what they were doing around stallions and were excellent horse people.
Best time is under a year, but you can any time in life. Latest I like to is around 3 - once they've experienced testosterone, they get a lot of bad habits from it. A weanling is DEFINTELY not too old.
Good luck.