Don't just go out buying lenses without knowing what you're getting yourself into.
Consider the nifty fifty (50mm f1.4) if you want to achieve that. 18-200 is more like an all purpose travel lensOriginally Posted by Melvintan244
Canon EOS 600D | BBB virus carrier
What you are trying to achieve is called subject isolation using background blur. Even the 18-55mm kit lens can do it, but it's certainly easier at longer focal lengths.
Remember: There is no such thing as the "bokeh effect".