Hi, you can connect an A500 to a modern tv in 3 possible ways:
The first (and best) would be a lead going from the RGB output on the Amiga to the scart socket of the tv. A dedicated lead will also allow for the audio output too.
Second, if the tv has phono socket inputs for video/audio, you can use the composite video out from the tv modulator and the audio out on the Amiga to connect. (3 phono leads needed - video and audio left + right)
Lastly you can connect through the tv ariel socket and tune the tv into the signal. Picture quality will be far worse than option 1, if I remember correctly!
Hope that helps.