How do you deal with grief?

Allow yourself to feel it.

There will be different phases; shock, numbness, bargaining, anger, acceptance.
There's no set order; any of these can come and go at different times.
Gradually acceptance and peace settle in.

Let yourself think about the person, all your memories of them, both good and bad.
If there were regrets, let yourself learn from them.
If possible, share the memories with others who also knew the person.

Consider writing about it, or making a small memorial out of photos and momentos.

Allow as much time as it takes - there is no set amount of time that is normal or right or wrong.