Log into the control panel for the website your ammending.
Go to Templates and then select the default template_group (i think it’s called “site”) from the left column. Then in the right column select the CSS file which I think is “site_css”.
In here you can change the you will need to find “font-family” for the sections your changing. If the font currently is Times New Roman you should see it written next to font-family. Write a suitable font name to replace the current one.
For legibility I would suggest Helvetica, Verdana or Arial (or put all three in separated by comma’s).
If your unsure of how to write CSS, this is what you will need.
font-family: Helvetica, Verdana, Arial;
I can’t really help you anymore than that.