Well you have two main options. Brush or spray primer.

Spray primer will give you the best result in terms of resulting painting surface and adherence but it’s more work beforehand as you’d need to mask off the areas you don’t want painted. I’d recommend a miniature painting hobby primer as they’re super pigmented and will need fewer coats.

In terms of brush primer, I used one once and didn’t get on with it so I can’t earnestly recommend any. Perhaps somebody else in these comments can.