By default, when you create a new blank document, the text you start typing will be a Normal style using a font of Calibri with a size of 11. This is a commendable choice, but it might not be right for you.
You can change that default. Start typing some text and then select it. Change its styling to be the way you want it. For example, you could change the font size, the font family and even the colour. Select it again and then right click on it and select Font.
When the Font window opens, click on Set As Default.
A reasonable question to answer is whether you want the new default font to apply only to the current document, or all documents based on the Normal.dotm template (to all intents and purposes all documents). You can answer that question in the next window that appears:
Make your choice and click OK. The chances are, you will want this new default font to apply to all documents, to save you having to change it every time.
What you have done here is change a style. In this case it was the Normal style. Similarly, you can change the styling for other styles as well (headings, subtitles, etc.) and make the same choice regarding application to only the current document or all new documents.