Showing only a summary of my posts and a photo thumbnail

Hello,

I have created a blog and in the home page it shows the lattes post entirely (the full body text and all the photos of each post). I want the home page (or category page) only to display a photo thumbnail and the first paragraph of each post and if the user is interested in reading more, s/he can click on read more and go to the full body.

Does anyone know how to do it? Can just this be done from the settings or I need to download a plugin (if so, which one)?

Thank you in advance for your time!!

MK

PS: I have heard that the Twenty Ten theme comes with a “Continue reading →” feature but I am not using that theme.