Please write me how to install linux 12.04 LTS stepwise. Am a beginner.

find a distro that will auto-format partitions for you, like Ubuntu (see above post), Fedora, Debian, etc... then you do not have to stress over it.

If you understand basic MS Windows partitions then here is a rough guideline that should be safe to use.

/boot no less then 1M
/ depending on your HDD storage, i typically try to give between 60G-150G
/swap (this is your MS Windows page file) will depend on the amount of ram you have. this is not cast in stone, but i do the following:
128M-512M RAM double your swap file to ram.
512M - 1.5G Ram double or match your swap to ram
1.5G+ match your swap to ram.
/home everything else unless you have multiple HDDs and want to put stuff out there.

Thats a real rough outline, not much different then MS Windows:

/boot = MTR in MS Windows you have zero control over this partition.
/ = c:\
/swap = page file (in MS Windows you have a file, not a true partition)
/home = d:\

Hope that helps.