Orcs are one of the most dangerous of the monstrous races that inhabit the Warhammer world; they are also one of the most popular armies amongst Warhammer players. The 'Orc' army actually encompasses many different creatures of which Orcs are but one type, so we often refer to the army as 'Orcs & Goblins' or 'greenskins'. We are going to use one type of Orc army as an example of how a player might put together such a varied army. The same considerations also apply no matter what army you choose.
If you want to collect an Orc army then you will need the separate Orcs & Goblins Army book. This contains extra games rules, more magic spells and items, and extensive painting and collecting tips, as well as the all important army list.
The army list is used to choose an army before each battle. Obviously, you will need to assemble a collection of models from which you can choose the exact force for a particular battle. Needless to say, most players prefer to build a collection around a typical 2,000 or 3,000 point army, but it's always a good idea to have a few extra models to give you some additional choice.
Before we start to look at the details of how to choose a force for a battle, it is worth pointing out that most players will cheerfully buy and paint a handful of models before deciding to commit to collecting a whole army. There's no point in buying hundreds of Orcs only to discover that you really can't abide painting green! So, whatever army you fancy the look of, buy a few samples first and see how you get on painting them. It doesn't matter if you dabble with several races before settling on the one you like best, it is far more important to choose the army that is right for you.