My grandpa is half-cree, half-black and he hardly spoke the language to me growing up. How can I learn the native language?

Wohoo! Go you!
 
For starters, if you haven’t connected with your tribe already – do that first. They best way to learn a language is to be around the speakers of that language.
 
If you feel comfortable and are able, reach out to your grandpa’s extended family. Tribal communities have a very strong familial bond. It’s likely they have thought about reaching out or at least thought about their relative but didn’t know how to. Let them know you’re interested in connecting and learning more about your heritage and language. If the family route doesn’t work, know that there are other opportunities to connect.
 
You can find information by conducting an internet search – a lot of tribes have official tribal websites where you can at least find a phone number to their tribal office. Give them a call. Most tribes have someone who can answer some questions for you. Ask for youth events like; language classes, youth groups, community gatherings, in-school or after-school programs, elder lead groups, pow-wow/ dance groups, or anything else you could get involved in. Get involved.
 
Once you start to make connections within the community and make an effort, things will open up, especially when you come with good intentions and respect. Keep this as your focus.
 
Best of luck to you. My elders say that when I speak my language (no matter how terribly), my ancestors hear me.
 
Best of luck!
 
Auntie Manda