[Theme: Twenty Ten] Sidebar fonts

First one should change the text in the text widget, second one should change the text in the others.

.textwidget p {  }
.widget-area, .widget-area a:link, .widget-area a:visited {  }

Don’t edit the Twenty Ten theme. Instead create a child theme for your changes.

You should start off by creating a child theme if you want to make changes to Twenty Ten. This will prevent your changes from being overwritten if it’s ever upgraded.

Codex guide – http://codex.wordpress.org/Child_Themes
My own guide – http://zeaks.org/child-theme-basics/

I crated a starter child theme for you, just upload and activate, then you can add all your changes to it instead of Twenty Ten.
http://zeaks.org/soulstyle-theme.zip

Once you have done that, open style.css and add the css to change the fonts. font-family changes the font, font-size changes the size
List of families http://www.fonttester.com/help/list_of_web_safe_fonts.html

This will change the font in the text widget

.textwidget p {
font-family: font-family: Arial, Helvetica, sans-serif;
font-size: 14px;
}

This will change the active font in the other widgets

.widget-area, .widget-area a:link, .widget-area a:visited {
font-family: font-family: Arial, Helvetica, sans-serif;
font-size: 14px;
}

Add them both to the style.css, and save it. Change the font to whatever you want, and the size to whatever you want. You can edit the style.css in Appearance > Editor