To download this Trial Version, you can go to: https://www.sdn.sap.com/irj/sdn/downloads.
Just follow the installation guide in the installtion package.
Here I give you the list of items that you are going to install with the installton package.
1) SAP Web Application Server
2) SAP Enterprice Portal
3) SAP Content Mangement and Collaboration (not included TREX)
4) SAP Netweaver Developer Studio
5) SAP NetWeaver Development Infrastructure
6) SAP Composite Application Framework including Guided Procedures
7) Interactive Forms based on Adobe Software
Based on my own experience, although it is only required 1Gb RAM to run the portal, but I suggest you to have 2GB RAM in your system to install this. As you will see some undesirable result and waiting time to get some things load and execute by have just 1 GB RAM.
(especially when you have about J2EE engine, database, 3-5 portal browser with heavy memory execution, and your Netweaver Studio up running).
Good luck for the trial!