One of my frequent answers:
You need to create your own default template. Open the blank template & insert a table, click in it & choose the font, size, color, etc. you want in the fonts panel or format bar. You can do similar changes to inserted objects & text boxes. While you're at it resize the box if you want, set the amount of extra space in the wrap inspector, etc. Delete each text box, object & table after you've set them. Now, save this document as a template & select that template as default in preferences. If you decide to make any changes to the template down the road, just Save As Template again with the same name & replace the previous version.
Now, save this as a template then, in Pages > Preferences, select this template as the default for new documents. All of your new documents will have these settings for inserted objects, images & tables. Unfortunately, the settings won't apply to any other templates or "foreign" documents you open with Pages.
About the only thing you can’t set a template to do is remember to show invisibles & the window position. It is possible to get a template to show invisibles when opened, if you want. Save this document as a document then, in the Finder, change the extension from .pages to .template & put it in your My Templates folder in HD > Users > (your account) > Library > Application Support > iWork > Pages > Templates > My Templates.Set up your document with the line spacing, margins, headers, etc. as you'd like. Redefine all of the paragraph & character styles to have your desired font.