Here comes the fun part, at least what I think is fun, to customize the theme, I will try to pick the most common and bascic customizations you might want to do with your theme.

So to get started, you have to go to your Wordpress dashboard, click on themes and click on the blue customize button on your theme. That takes into a new editor that allows you to preview the the changes on your theme as you go. I will be using my theme to show how to do the settings.
I must warn you, the diffrent themes come with diffrent options, so whatever theme you have installed might not look the same, but the principles are the same, so the basic should be covered anyway.

So this is how it looks like when I clicked customize on my theme

Change the colors of your theme

In customize mode look for Layout/Design option and click on colors, when inside click on select color and choose whatever color you want to have in your theme. As you see in my theme I have only two options so I have chosen a white background and a blue primary color, but you might have more color options to choose from.

Change the Site title and Slogan

You might want to change your title and your slogan, the text that appears under the title. Go to site identity and change to your liking, I have also a possebility to upload a favicon, the icon that appears in the browser.

Change which side your sidebar should appear

You can show your sidebar on the Right side, as I do, or you can have it to show on the left side or both, that is up to you. Go to Layout /design options, and click Sidebar layout. Choose which side from the curtain button.

Change which items to show in your sidebar, go to Customizing widgets, click the add widget button and choose which widgets you want to appear in the sidebar. If you want to place code in your sidebar, choose the text widget and copy and paste the code you need.

Notice, you'll have to click save before your changes is published, visible to everyone.

Congratulation, you have now changed the look of your theme, there is of course many more things that can be done to customize your theme, my recommendation to you is try out all the customizing features your theme come with and activate, customize those you like. You can't do anything wrong. Next and last in this basics tutorial I will show you how to add banners to your site. Keep rolling almost there.