Within Google Drive, click +New
Click Google Slides
Click Blank presentation
This will open up a new blank slide deck that you can begin working on.
NAME your presentation with a suitable name that reflects the subject of your slides.
When you open a new slide deck the themes appear on the right hand side. Click on the theme that you think would be best for your presentation. You can always change this later. Click on the X in the top right hand corner of themes when you are happy with the theme.
If you decide later on that you want to change your theme, simply click on:
Slide
Change theme