You should start w/the SmartThings Smart Lighting app - runs locally which makes it reliable and provides fast response, covers many use-cases from very simple to moderately complex, and is simple to set up and use.
If you want a free option, there are a couple of alternatives. For ones developed by the community here, look at the quick browse list in the community – created wiki, in the Project Reports section, and look on the “dashboard“ list.
I am kind of lost. That will make automation. I just need an app, like Phillips hue, where you can turn on/off individual lights and switches. And you can group lights to operate whole room and house. I don’t want extra stuff, like motion detection etc, just simple on/off app.
If all you want is lights, I’m not sure you need smartthings at all, to be honest.
If you have an iPhone, look at HomeKit. It works with both bulbs and switches (including the Phillips hue bridge and Lutron Caseta switches), it’s intuitive and easy to use, and if you want to set it up like a dashboard you can. You can do simple automations with it like having some lights turn on at sunset or when you get home, but you don’t have to. It’s reliable and easy. But you do have to have both an iPhone and either an iPad or HomePod or an Apple TV 4.
If you use android devices instead, you could get an echo Dot and just use the Amazon Alexa app. It’s super easy to turn lights on or off or group them, and you have quite a few choices of switches (again, including Lutron Caseta, but also a bunch of Wi-Fi switches). It doesn’t make as nice a dashboard as HomeKit, and you really can’t set up any rules except a time of day rule, but it’s certainly an option for some people.
HomeKit is great and so is SmartThings, and the two can both control the same devices. Then you can use Siri and an easy dashboard interface, while still getting the advanced features of SmartThings.
If you install homebridge and in particular homebridge-smartthings on a desktop you can even have HomeKit control things via SmartThings, which opens up many more possibilities for HomeKit. You can get the best of both worlds on your iPhone or iPad. I spent years agonizing on what home automation platform I should buy, now I realize that two (or more) can work together.
Unfortunately installing homebridge on your desktop is a bit advanced, but there are some good tutorials.