just bought a 2003 accord
 

When I start the engine it sounds like a belt is slipping. I don't know if it has ever been replaced. How many miles before the belt should be replaced? And how difficult is it to replace by myself?

Normally the squeeling just means it needs to be tightened. Difficulty will depend on what belt it is, not sure of the exact set up on that year.

Is this a 4 or 6 cylinder and how many miles on it?

Both 4 and 6 cylinders have a drive belt (serpentine belt). There is an auto-tensioner that has a gauge on it. Generally, that belt change interval is around 105,000 miles or 7 years, whichever is first. However, the tension gauge position should be checked as well as the condition of the belt for cuts, chunks missing, etc.

If there is noise, it may be the belt; but, sometimes it is also the tensioner.

The drive belt isn't too hard to change.