There are converter boxes that claim to give you a Composite Video signal from VGA, but I see varying reviews of how well they work and of their video quality. Maybe another way is to buy a USB-connected video interface. Some of these boxes may simply be "capture cards" aimed at getting Composite Video and S-video signals into your computer for capture to digital files, but the may include Outputs of these signals, too. Some cards, on the other hand, are more complete TV tuner cards able to tune in actual broadcast (or cable) TV signals, and some of those also have at least Composite Video outputs.
Assuming you can get Composite Video (or S-video) outputs from your computer, then you still have to get them into your TV. Many TV's have Composite Video (and companion stereo audio) RCA input jacks for this. Some may have S-Video inputs. BUT if you have neither, then you have to find a way to convert one of those signal formats into a modulated TV channel signal on Channel 3 or 4, and feed that into the TV antenna input connector.