Firstly, to post a new topic, you click on the link to the board that you want to post in: this post is on the babble board, but since you need help with installing custom content a moderator will move it to the technical help board, I think. At the top right hand side of the list of topics, there is a green button that says 'new topic'. Click that button and then you can type your message.
Secondly, there are two types of files that you can download custom content as. Where are you getting your custom content (CC) from? If you download a sims3pack, all you need to do is open the folder that you save it in, and double click the file. It should have a little sims icon. The sims 3 launcher will open and install the item - simple as that.
If you downloaded a .package file, then you need to put it in My Documents/Electronic Arts/The Sims 3/Mods/Packages. If you need further help with how to install .package files, there is a good tutorial on modthesims,
here.