Capture form input from text_field and modify data

I'm new to ruby and just trying to toech myself ruby on rails. I've
got a shopping cart and I need to be able te specify product quantity
while in the cart. I have succefully created a link to incriment the
quantity but I would like to be able to grab a number from a
text_field and use it as the new quantity. The incriment feature is
implimented as follows:

This is in my view:
<%= link_to("<font color=green>(add)</font>", :action
=> :update_cart, :id => %>

This is in my store controller:
  def update_cart
    product = Product.find(params[:id])
    flash[:notice] = "Item quantity has been updated"
    redirect_to(:action => :display_cart)

This is in my cart class:
  def update_product(product)
    item = @items.find {|i| i.product_id ==}
    item.quantity += 1
    @total_price += product.price
    @total_shipping += product.shipping

I'm clueless here and could really use some help. TIA