I'd has 3 meanings: 1. I had eg I'd better go home now 2. I would eg I'd love an icecream 3. I should eg I'd say goodbye, but I'll say see you soon.

you are right that it means " i would", "i could", or "i had ". you should use "i would" if you want to say what you will do in the future like: i would marry someday. use "i could" if you want to say something you can possibly do or something that you can do before like : i could paint a house. and use "i had" to mean what you have in the past.