When does something become a Sin

Salaam I want to know when a sin becomes a sin, for example I use 2 eat a lot of food that contained gelatine like sweets which is Haraam. Even though I havent eaten something like that for a long time i still think it tastes nice & I enjoyed eating it. so just having the thought of liking/thinking about something Haraam even though you know its Haraam, Is that a sin or is it only a sin when you actually commit the act. Jazakallah