Stored Procedure To Get All Users Or All Users Of A Specific Country

hi all
i've got two tables called "webusers" (id, name, fk_country) and "countries" (id, name)

at the meantime, i've a search-page where i can fill a form to search users. in the dropdown to select the country i included an option which is called "all countries".

now the problem is: how can i make a stored procedure that makes a restriction to the fk_country depending on the submitted fk_country parameter?
it should be something like

SELECT * FROM webusers
(if @fk_country > 0, which is the value for "all countries")
{
    WHERE fk_country = @fk_country
}

who has an idea how to solve this problem?