I wouldn't start with "The Colour of Magic", it is - by far - not the best of the Discworld novels, and tbh I find Rincewind a tad annoying.
I personally find any novel in the Witches arc a lot more enjoyable, starting with "Lords and Ladies". Other good ones: "Carpe Jugulum", "Equal Rites".
Another very enjoyable set is the night watch arc. Both "Guards, Guards" and "Night Watch" are excellent.
[edit]: oh yeah, and how could I forget: "Small Gods". Hilarious even if it stands a little aloof of the rest of the discworld stuff.