Familiarisation? Probably the easiest place to start is the Activity Monitor.
Watch for Recent Expensive Queries and examine them in detail as they come up. Experiment by sorting the columns - click on [Executions/mi] to see frequently-run queries. Hover on column titles to see which DMV is called to obtain the values.
Keep an eye on Waits in the Processes pane, and obtain more information if necessary from the relevant DMV.
Don't forget indexes. Glen Berry [/url]publishes a suite of diagnostic information queries on his website - they are tremendously useful for this type of investigation and include index investigation for refactoring.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden