What rate to charge: What seems reasonable to you? Ask for that. If you get no takers, you're asking too much. That's how prices are set in a free market economy.
How to get advertisers to find the contact page: The target market for the advertisers are your readers. Some of your readers will also be advertisers or know others who will be. That's where the old-style "Your Ad Here! Call 555-1212 for information" boxes come in.
There you go. Best of luck to you.