I download a Joomla quick starter package and when installed it..
I found this error...anyone can help please?
Fatal error: Allowed memory size of 25165824 bytes exhausted (tried to allocate 323588 bytes) in \libraries\phputf8\mbstring\core.php on line 122
The error is showing your page is trying to load more than 25 Mb into memory. This is way too high, a normal installation with a few customizations usually doesn't go over 5mb, and can be as low as 2 or 3 depending on what you're showing.
I am assuming you can access the administrator interface at least. From there, turn on System-Debug in the Global configuration, and reload the frontpage.
If the page loads, at its bottom you will see the breakdown of the memory consumed by the different modules, most likely there will be only one using up all memory.
If the page doesn't load, since mbstring is not used by Joomla itself, it may be worth trying with another template: all the positions will be wrong but you could be able to see the page and try to debug it. Just go to the extensions-template menu in the administrator and choose beez or another default template for the frontend, reload and continue with the debug.
If the page still doesn't load, try to make another page the default page, and on that page just put less content and fewer modules. You would do that from the menu manager, just click the star next to a menu item and make it yellow. Then load the frontend and check memory.
A quickstarter should have been tested to rule out such high memory consumption, so a final option is that it gets stuck on some legacy code, which you could find out examining your apache's error_log. Finally, you might want to ask the developers for support or try reinstalling the quickstarter without any data in and work your way up.