As long as you don't out-dress the bride and groom, why wouldn't you go to a wedding looking your best?
If you don't have a suit you can wear or are too broke to buy a new suit (you should have at least one working suit anyway for ocassions such as these), then at least have a blazer and some slacks. A navy blazer is okay as long as it's with contrasting pants, like grey or a cream colour. For a shirt, you can never go wrong with plain white. Get a pair of interesting brown dress shoes and you're good to go.
If you're interested in standing out, you can wear a pocket square with your blazer or flash some cuff links. Going belt-less is okay - it takes the guesswork out of finding a matching belt and shoes.