LingTree is a relatively simple program. It comes with two resources, both of which are accessed via the Help menu item.
The full set of help topics is at Help / Help Topics. Please read this (relatively short) document to learn about what LingTree can do.
The second resource is a Quick Reference Guide. This is a summary of what to key in order to get a certain effect in the tree diagram.
Both of these resources will stay showing while you work on a tree diagram. This is an attempt to make them handy for quickly seeing what you need.
The program also comes with some sample tree files. On 64 bit computers, these are in the C:\Program Files (x86)\SIL International\LingTree\Samples directory. On 32 bit computers, they are in the C:\Program Files\SIL International\LingTree\Samples directory.
You can also see the LingTree Helper web site for another way to key tree diagrams that some users find more intuitive.