Usually, when I look at the plan, I see to find any irregularities or things that I don't expect. The main question for me is: what are the most costly parts, does that make sense or should I try to reduce it?
I'm not aware of any tutorial or documentation on how to interpret the plan, perhaps it just boils down to experience and fiddling about?
There is lots of documentation about interpreting plans, but to get the most out of them you also need quite a lot of rounded knowledge of SQL Server, T-SQL and a good understanding of what you expect to see and what would be a warning.
For example, table scans and hash joins might be fine. However, if you see these and your query is designed to return a single row then you would want to investigate. Conversely, a seeing lookups\ seeks on a table where more than around 5% of the table is returned is also of concern.
However, as a starting point I would look at the percentage cost of each node. The most expensive ones are the ones to focus on. As you focus on a node, google it (BoL IIRC is not very good on this topic) and learn what you can.