Keep all the police records of your arrest etc - when you apply for a visa for certain countries, you'll need your police certificate. It will say "no live trace" - and that shows you have had something before (it will say "no trace" otherwise). For example, getting a visa to the US, if it says "no live trace", you will have to provide a detailed report which will include spent convictions too.
You will have trouble getting a Canadian visa with a DUI, other countries (including USA) are fine. And yes, you will have to declare it.