Whenever I want to install a new package with yum, It first checks for updates. Is there a way to stop it from checking for updates each time a package is going to be installed?
Please define "checking for updates" ... do you mean that it checks if the repo. metadata is valid? You can alter that with the metadata_expire variable (see man yum.conf), but in general I would not recommend running against old data.
If you mean the obsoletes checks, you can turn that off ... but again, that's not recommended.