What can you tell me about the Yokosuka Line?
If you are planning to go to Yokosuka city from Tokyo or Yokohama, it is not advisable to use the JR Yokosuka Line. Use the Keikyu (Keihin Kyuko line) as it will be cheaper and much faster (about 20 minutes faster).
The Yokosuka station for Keikyu is called Yokosuka Chuo 横須賀中央. Be careful to get on the right train as Keikyu has branches that split off going to Haneda airport and Zushi.
If you are going to Kamakura or Zushi, you can get there on the Yokosuka Line. Check your time tables as the frequency of trains is not so great. (Look for an express train heading towards Miura Kaigan 三浦海岸 or Misakiguchi 三崎口 and you'll be on the right train.