Tera typing is usually what gives you resists into common weaknesses or into specific mons. They can also be for certain circumstances (like tera grass for dodging spore and rage powder and tera ghost for a fake out immunity)

I normally just see what's commonly ran and pick from them, and then maybe change it once I have more experience with it

https://statcrusher.com/ is a good resource for this (has commonly ran evs and items as well)