download Mavericks at app store, can i install? I have downloaded Mavericks from the App Store. Now I have the installation image, but I am afraid to install it because it might delete my files and programs under Mac OS X Lion 10.8.4. If I install Mavericks, will it all be deleted?
No it won't delete your personal files or programs you have installed on Lion. The installer will upgrade your Lion installation to Mavericks and will keep your existing programs by default.

However, it is always a good idea to create a backup before upgrading your system. Just in case somethings happens during the installation or in case some of your software is not compatible with Mavericks. The backup will allow you to revert backup to Lion. I suggest you use TimeMachine with an external hard-drive to create a backup of your entire system before upgrading it.

Macworld has also written a great guide about what you need to know before upgrading to Mavericks.