I installed oracle11g but after installation my system become very slow, now What remedies you can suggest for my system?

If you want to use oracle for practice purpose use orcle express edition rather than using full version.

mainly review your installation

the processor cash

the ram of your system

any data base will share a part of your system memory

so if you have some lagging, so mainly your system memory is diversed in another process

and also ,, some lagging may be due to what you are doing

reports, calculations,, etc

just analyse your system resources to find what eats your system memory

Provide the H/w details of your system and OS version using are running on.

There could be many reasons of it. I am not aware about the operating system, here I am using Linux or any Unix version.

Check the current running process and OS conumptions

top

sar

free -m

uname -r

uname -a

cat /etc/issue

cat /proc/cpuinfo

send me the output of these commands, may be  I can provide some help to you.

Thanks

what is your system type i.e OS,memory,processor ,configuration for SGA??