Here is an official tutorial on creating artificial depth of field.
Please check the site before posting a question.
If you shoot with the intent of adding depth of field in post, by shooting the elements seperately, it isn't too difficult to achieve. If you have a finished shot already, and you want to add depth of field to it, then it will require a lot of frame-by-frame masking to cut out the foreground object on one layer, then apply a blur of some kind to a duplicate background layer. Defocus is the filter you want to use ideally, but you need Compositelab for that. Try the blurs included in Elab Pro to see which one you are happiest with.