To use a SSTV software program (and many other modes) you will need some sort of interface between the PC and the transceiver. There are quite a few commercially available interfaces, some kits and some ready to go units. These vary in cost but are becoming less expensive all the time.
However, there are also many circuits available if you want to try to make one yourself. Below is a very simple design which is worth a try if you have some time and some bits in the junk box
Resistor values shown are shown in a T pad configuration and attempt to roughly match the input and output of the PC and the rig. The values of the RX side are for audio taken from the line level out of a rig and will need fine tuning.
The rig input shown is for using the mic input of the rig, values will need to be adjusted for your rig but shown for a rig with 600 ohm mic input.
To reduce hum and noise from earth loops and stray RF, I used a pair of Ground Loop Isolators from Jaycar, CAT. NO. AA3084, with the addition of some clip on ferrite noise suppressors. You may or may not need these depending on you own situation.
The PTT circuit shown is an open collector type which some find works fine, however, I like to add the relay which isolates the PTT circuit and gives a good positive PTT. If a low current relay is used, the 5 volts can usually be picked up from one or more of the unused high control lines of the serial port. The transistor is not critical, many NPN transistors such as the BC547 can be used. Likewise, many diodes in your junk box will do the job.