If its a direct email, then you should find the address in the email header. If its an email address via say a webpage, then this should be provided if its a commercial website. if it isn't, then you might need to contact the originator direct via telephone etc (if telephone or direct mail address was provided).
As already been suggested, perhaps more clearer information from you, might help in resolving the problem you might be having?.
As per Nontek asked, which website is it? Copy the url and paste it on here so we can go to it and tell you what you have to click on. It may be a direct 'mailto' link or perhaps first you have to click on 'Contact Us'. There are various ways that websites display there contact details. Or if it is a forum like this one that does not display members email addresses and also does not have a PM private message facility you have to put your request to the Forum Editor who will decide whether to pass a message on or not.