Changing the footer text

One possible way to hide footer text would be to use a CSS editor such as the one that comes with the Jetpack plugin: http://jetpack.me/support/custom-css/

Here are some steps you can try:

1. Install the Jetpack plugin
2. Enable the Custom CSS module
3. Go to Appearance > Edit CSS
4. Enter the following to the CSS editor and save changes:

.site-info {
	display: none;
}

To insert plain text, you could do that with CSS using a rule like this:

.site-footer:after {
	content: "Put your text here."
}

To decrease the height of the footer, add something like this:

#colophon-wrap {
	padding: 30px 0;
}

Adjust the 30px as needed.

You’ll need to make those modifications on your own. I can help you get started!

Here are a couple great tutorials about CSS if you’re just getting started:
http://www.htmldog.com/guides/cssbeginner/
http://www.codecademy.com/courses/web-beginner-en-TlhFi/0/1?curriculum_id=50579fb998b470000202dc8b

This guide shows you how to dive in and pick out which selectors will work best:
http://en.support.wordpress.com/custom-design/how-to-find-your-themes-css/

You can also look in the theme’s style.css file, copy the relevant bits to your CSS editor, and make adjustments from there.

Regarding changing details of the text, it matters exactly what text you want to target. To give you one example, this would make most of the text a little bit smaller in size:

body, button, input, select, textarea {
	font-size: 15px;
	font-size: 1.5rem;
}