Just to add...
If you created and saved your game using TIC's editor it'll already be in .tic format (unless you saved it as something else eg .lua). If you did save it as a .lua or other file, just load it and save it as a .tic from the console, eg load mygame.lua -> save mygame.tic
As MFauzan says, typing 'folder' from TIC's command line will show you where all your carts are saved/exported.
For more help with 'export' see: https://github.com/nesbox/TIC-80/wiki/Console
...or just type 'export' in the console :D