There are several ways to accomplish this, but it is highly dependent on the type of website you have (Wordpress, normal html site, etc.) and if you don't have a good knowledge of html, you should probable engage your webmaster or do a google search for additional help.
With that said, the following code may work for you. You put this right after the <html> tag of the main page (normally index.html) of your website.
if (/Android|iPhone|iPad|iPod|IEMobile/i.test(navigator.userAgent) )
Change the url brand/app to your brand and app that you want to redirect to. This will cause people visiting your main website to automatically redirect to your MyPhotoApp mobile website if they are using an Android, IOS, or Windows based mobile device.
We do not warrant that this will work nor do we provide support for your website. This information is provided as a courtesy to MyPhotoApp users.