Sometimes there will be contact information on the page, other times you'll need to use the contact in the WHOIS information. This should still work even if it's been redirected through some privacy feature of the registrar.
I've not used them, but I think escrow.com is supposed to be able to handle the process of transferring domains when the person you're dealing with is not someone you already know and trust.
For an offer price, it's really up to you to just do some research and use your own knowledge and experience of what domains might be worth. Note that domains are often advertised at highly inflated prices and a lot of domain appraisal services also give inflated prices. Don't offer too much, but don't be too close to just the registration fee. The rest is, I'm afraid, up to you to research. I will say that in any negotiation, the most important way to avoid being screwed is to always be prepared to walk away if you don't get the price you want. Sounds simple, but some people really feel like they must have something, so they'll pay a lot more than they would have otherwise said was their upper limit.