Which job would take the least amount of training?

I'm looking for a job that requires the least amount of learning since I am only looking to have this job for a couple months of the summer. I feel like working at a restaurant would take a lot more time to adjust to than 3 months, but i'm not sure, since I have no experience in that. I was thinking about Hastings, Ross, Target, Walmart, Goodwill, lalala... any options for a highschooler that lives in Helena Montana :)