problem with custom fields

I’m trying to insert a custom field to each post on my homepage. The custom field is to display a thumbnail image to the left of the summary of the post.

After reading and reading official and unofficial documentation on custom fields and testing I have not actually doing so.

I added a custom field to the post but I fail to understand what the finished code I have to insert my index to display everything and where to insert the code in the index.

Let’s see if someone can clarify.