Tree diagrams
or
  phrase markers

Before you start diagraming a sentence it's recommended (not to say necessary) to analyze it using a syntactic viewpoint, which is simply to identify the different phrases and/or clauses found in any given sentence and how they can be represented. This is an important aspect of Parsing that is another way to call this kind of analysis. 

Taking into account the different types of phrases (noun, verb, prepositional, adjectival, adverbial), look at these sentences, perform a syntactic analysis and diagram them. 

1. The students in class are not paying attention. 
2. A new has come to class with harder activities. 
3. We could've decided faster.
4. My brand new tv was really expensive in this store.