What ISO should I have my Canon A1 set to?

The ISO for that film is 200, hence the the name. Most film will have ASA / ISO rating on box and some include the ASA / ISO rating in their name.

200

I would thought the lower the better. Depending on your light condition, you wont want to use high ISO. For my Sony dslr , using is above 1800 will cause the picture to look grainy.

Golden rule. As @mea05key said. The lower the better.

Lighting conditions can always be corrected with a flash and a tripod.
I do my best to avoid anything over 100 ASA.

Bright and sunny – low ISO (100–200)
Partly cloudy – medium ISO (400–800)
Sunset – high ISO (1600)
Night – You’ll need a flash. :)

The higher the ISO (800–1600), the lower the quality and the more grain you’ll get on your photo which is why it’s best to stick to the lowest ISO.