Whats the deference between i like to be a genius and i like to be genius ?

In your first statement genius is used as a noun. in the second one genius is used as an adjective. You cannot say 'I like being a smart.', because you are using an article 'a' and you must have a singular noun to ociate your article to. In your statement 'I like being a smart.' you only have 'smart' as an adjective after the article 'a', and not as a noun. Unlike 'I like being a smart guy' your article 'a' is pointing to the noun 'guy' making it grammatically correct. If you want to remove the word guy you have to drop your article as well so you can say 'I like being smart' instead of 'I like being a smart'.