Your installation of Ubuntu 11.04 has reached end of life, so you can no longer update it or install packages from the default Ubuntu repositories. I strongly recommend installing a supported version of Ubuntu. Ubuntu 14.04 is a Long Term Support release that will be supported by updates until April, 2019.
Ubuntu 14.04 has different systems requirements than Ubuntu 11.04. Your computer should have at least 768MB RAM, preferably 2GB RAM for decent performance.
You can download Ubuntu 14.04 from the official Download Ubuntu Desktop webpage. After you have installed Ubuntu 14.04, you can customize it to get that collection of Mac-like skins and themes that you currently have by installing the MBuntu Transformation pack for Ubuntu 14.04. The theme of the MBuntu Transformation pack for Ubuntu 14.04 resembles Mac OS X Yosemite.