My WordPress Theme blank screen?

I have created my own wordpress theme with all the necessary files, however I don’t know why it comes up with nothing when I activate it? Literally a blank white screen. It doesn’t show up with anything in the preview either.

I’m also having problems testing it locally – I have converted my html code into the correct wordpress theme files, however when I open my index.php file in my local server, I get a blank page. But my html version still works… is this normal? How do I test the theme?

Very confused and hope to find answers!