Previously, I had a basic touchscreen phone (not a smartphone). It used a regular SIM card.
Then I purchased a cheap smartphone and placed this SIM card into it and it worked.
In the near future, I will be purchasing a smartphone that uses a microSIM card (Nexus 4).
What do I need to do? Do I go to an AT&T store and purchase a microSIM card for my 10 cent per minute GoPhone plan? Or do I buy a microSIM card cutter?
You could do either. Personally, when I switched from using an Android smartphone with a regular-sized SIM slot to an iPhone 4 with a microSIM, I didn't want to go through the hassle of going to an AT&T store to request a microSIM because it probably would have been a waste of time. iPhones were "not supported" for use on prepaid plans around this time last year. They recently just changed the policy about a month ago, actually.
In this case, I bought a cheap microSIM cutter. It was one of the easiest things to do. It cost around $4 and I only used it for a couple of seconds, but I thought it was well-worth it. My standard-sized SIM was transformed into a microSIM within 10 seconds. I would highly suggest purchasing a microSIM cutter.