The purpose of flying a holding pattern is usually to delay the aircraft in the air while keeping it confined to a specific airspace. It is the aviation equivalent of stopping a car at a traffic light for some time.
The details of holding procedures (entry procedures, speed limits, obstacle clearance, etc.) are described in the ICAO DOC 8168 (Procedures for Air Navigation Services - Aircraft Operations, Volume I, Flight Procedures).
There are many ways to determine and to visualize the entry to a holding pattern. Take a look at these tips about entry procedures for holding patterns.
The mobile application allows you to practice entering holding patterns. Have fun!