depends on the BT adapter but it is not "that" simple ...
in general
1. you need to write routine on your uC to pair BT module with PC (on some button press or whatever)
2. before you start - you need to "initialize" BT adapter
so in general this means
1. you need one C function for pairing (cca 30lines of code)
2. before you start using the rs232 - you need to init BT module (2 lines of code)
that's it for most of the modules ...
there are some modules (bit more expensive then 50$) that do automatic pairing and they automatically establish serial line so you do not have to do anything on the pic side ... the problem is you would have to have 2 modules, one on the pc side and one one the uC side
I'd recommend you use ZIGBEE modules .. they cost ~same, complexity to modify your application to use them is ~same, you will have to get 2 of them (one for PC) ...
in any case, on your uC side you use max232 to change levels from uC to DB9 and vice versa. When using modules (BT or ZIGBEE or ..) you do not need to shift levels to 12V... You might need to shift levels down to 3V (for example parani esd bluetooth module is 3V)