Go to the form, click edit properties and then select "Use Workflow" to specify a workflow to use on form submission
Or alternatively you can use the code below directly in the code of your form if you need to be able to choose from a range of workflows that can be triggered when user submits the form.
<select name="WorkflowList" >
<option value=" - id of workflow 1 goes in here - ">
"Option 1 Text"
<option value=" - id of workflow 2 goes in here - ">
"Option 2 Text"
I thought there was an article in this inside the documentation on this but i'm afraid I couldn't find you the link.
Best of luck