The first thing to decide is what device you want to use to make the outlet itself controllable by SmartThings. You can either replace the outlet with an outlet that has a radio inside of it, or you can use a plug-in “pocket socket,” which is what many people do just because it’s easier.
We do need to know what country you live in, as the device choices do vary somewhat.
There is a discussion of plug in pocket sockets in the following thread (this is a clickable link), but those are all for the US:
Once you have a way of turning the outlet on and off from SmartThings p, now you just need a device to act as a wall switch. There are quite a few options for these, both mains powered and battery operated. You can take a look at the buttons FAQ. It lists devices for both the US and the U.K., both handheld and wall mount: