Image Attachment as Thumnail on Index.php, Full Size on Post

I am fairly new to creating WordPress themes, and I am sure there is a simple way to do this, but I don’t know where to start. I want to attach an image to my posts that displays as a thumbnail for each post on the main blog index (with a default image when none is attached), and the same image displays full size in the single post template.

So, what’s the best way to do this? Does it require a plugin, or can I do it without a plugin? If it requires a plugin, are there any good ones that you would reccommend?

Thanks in advance for your help!