First of all, it wasn't clear from your question if you have CDH or stock Apache Hadoop, HBase, etc. installed. That's important - although it will theoretically work on stock Hadoop, Impala is only tested and supported on CDH.
If you do not have Impala or CDH installed, by far the easiest way to do that is via Cloudera Manager, which will automate the install/deployment of a CDH/Impala cluster. Cloudera Express, which is free, includes everything you need to do that. You will have the choice of doing an automated single-package install, or by downloading a series of Linux packages. The options are described in detail here.
Or, if you're just looking for a demo, download and install the QuickStart VM, which contains a single-node cluster (including CDH + Impala), guest OS, and data/scripts/examples.
Downloads for any of the above can be found here.