By taxi from Keisei Narita station, you're probably looking at 10 minutes. Best print out a map from the onsen web site. The taxi may be expensive - probably between 2000 to 3000 yen. There's also a train route described on the site, but I think that might be more than you want to deal with on your first time in an unfamiliar place.

Prices for bathing are on the web site. Basically, the admission fee is the only compulsory one. If you bring your own towels, you're not obliged to buy them on site. Bear in mind that most Japanese people carry a rectangular towel (about 50cm X 20cm) which serves to preserve modesty while walking around the bath and to scrub the body in the shower. If you squeeze the water out of it, you can also use it to dry yourself. Saves you the cost of a bath towel. Shower well before entering the bath and make sure you're not soapy when you get in. Put the towel down on the edge of the bath when you climb in. Don't "wear" it inside the bath.

You'll probably spend less than 30 minutes bathing. The water is very hot, so one tends to climb in and out a couple of times. There's a sauna too. But all told, you're looking at less than an hour from entering the changing rooms to when you come out looking like a boiled shrimp and head for the beer vending machine.

Lunch menus are on the web site. The first item on the Ajisai menu is a 1500 yen course that looks good. .

The staff should be happy to call a taxi for you when you leave. In case of language barriers, just write down "Keisei Narita Station" in English.