You need 'besides' (with an 's' on the end).

They are two different words. 'Beside' means 'next to', as in "He sat down beside me". 'Besides' means 'in addition to', as in 'What are your interests, besides politics?' It's not clear what you're trying to say in your sentence. Do you mean 'Let's talk about something besides this topic'? That's possible, but it would be more natural to say 'Let's talk about something other than this topic'.