It's not easy to catch onto this idea about capital being a social relation mediated by things. People on TV don't talk about it like that, and nobody else is going to reinforce this for you. You have to spend some time thinking about it, and you have to make yourself come back to the idea whenever you lose it.
Based on my own experiences, I would say that the capital-relation begins with what we choose for ourselves in school, and ends with a lifetime of walking in and out of CVS and Target stores (popular retailers in the US). Whatever career we choose, and whatever things we buy are less important than the fact that we divide our time between working and consuming in a uniform way. If you look around at just about anyone in your life you will find this commitment to work and consumption to be relatively constant, while time for other things is apportioned with greater variability, if at all.
The things that mediate our relations with other people are called commodities, and capitalism is a system based on commodity production. A commodity is just a commercial product. It's something that is made and sold, as opposed to something that is made and used, without commercial exchange.
If you want to get a good idea of what our lives are about under capitalism, just walk into a CVS and ask yourself what might be done about a homeless person soliciting customers in the parking lot. In spite of the fact that the place is stuffed with commodities of every stripe, there's nothing you can buy that will change your relation with this person in a lasting way. What can you do but deny that you have any relation to them -- even if you do give some change? Many of us learn not to acknowledge such a person at all.
A lifetime of walking in and out of CVS stores might get us the commodities we need, but it doesn't address social problems in a lasting way. If work and consumption are our primary activities, then social problems become secondary considerations. The fact that they persist becomes "evidence" that you can't do anything about them, instead of being the predictable outcome of relations that don't do anything about them. This stems from the fact that our relations with others are mediated by things that don't help us in areas of mutual concern, because they are assigned to a different purpose entirely.