Your question is not clear as choosing keyword means a lots of thing. Like you have mentioned that you do wedding site, i think you mean you have a wedding site. Now the question is whether you want to optimize it locally of globally. What type of services you provide i.e your key services, their search volume and keyword competition. After you get the answer of these questions then use adword keyword tool as suggested by others. But before that decide what type of marketing is your requirement.
yeah, "wedding side" alone is a pretty big word. it is a market, not a niche. probably you may want to zoom in a little bit. as mentioned by kash, you can use keywordtools to get the ideas. seriously this is a big subject, they are many more factors that you need to consider ie target group, competition etc.
From the link provided by kash_2000 you can use the "Find keywords from Website" function to see which keywords can benefit you. "Wedding" is a very general keyword. You can break it down to gowns, accessories, banquet hall, travel, planners etc...