I downloaded an ubuntu image file, but its size is 30 MB more than a CD can hold. Also, my computer can not boot through a DVD or USB drive; what should I do? Please help!
Really you have multiple choices:
First of all you can try a bigger Cd capacity(there are 800 MB CD's)
You can try The Network installation or Install directly from Internet.
Please check this https://help.ubuntu.com/community/Installation#Alternate_Installation
Here is a list alternate installations so read and choose the one that helps you
Install 12.04 using Wubi
Note: This is if you running windows.
Ubuntu 12.04 will run from CD. See Ubuntu download page here:
A "700MB" CD would be a good start. They can fit 737MB on (because the advertised number is actually a MiB) and even then there's often some overburn space. I've got to over 750MB on a 700MB disk
After installing 12.04 either you can upgrade to 12.10 and so on or wait till April 17 and you can directly upgrade to 14.04LTS