The first question you should ask is whether you need to install the DiscoveryService. If you're working in a non-Java environment, you may find it easier to build a selection page in a more native fashion, or you may find that the SP alone provides enough rudimentary support to get started.
If you do decide to install it, you'll primarily need to collect the metadata sources that will contain the IdPs that users will select from. If you're planning to use SAML 2.0 or other protocols not supported by the old WAYF model, you may also need to provide metadata about your SP(s) to enable the DS to safely interact with the SP.
The Discovery Service will automatically select between "WAYF-Mode" (in which it intercepts and handles a legacy Shibboleth AuthnRequest message) and the full Discovery Service Protocol.
No explicit configuration to select the right protocol is needed.
The Shibboleth Discovery Service, version 1.2.1, is a standard Java web application.