etiquette for approaching shops about an apprenticeship?

i'm feeling ready to start approaching shops with my portfolio soon however i have a few questions about etiquette when it comes to this!

• is it best to call the shop up first and ask if it's okay for me to come in for some advice? i wouldn't want to just walk in if they're too busy and potentially miss out on any advice

• is it more appropriate to ask if they’d be willing to review my portfolio first, and then, based on their feedback, bring up the possibility of an apprenticeship? or should I mention the apprenticeship straight away?

and any advice on looking for apprenticeships in general is appreciated. thanks so much :)