error messages i do not understand during a build of android project

I have searched the forums, but none of the solutions proposed cured my problem. I cannot compile my project. I get this error, followed by :

Description Resource Path Location Type
Project 'videothumb' is missing required source folder: 'gen' videothumb Build path
Build Path Problem
The project cannot be built until build path errors are resolved videothumb Unknown    
Java Problem
Unable to resolve target 'android-5' until the SDK is loaded. MojoGBAMulti Unknown    
Android Target Problem
Unable to resolve target 'android-5' until the SDK is loaded. MojoGBAMulti Unknown
Android Target Problem
R cannot be resolved image.java /ImageThumbnailsActivity/src/image/Thumbnailsline27    
Java Problem
R cannot be resolved image.java /ImageThumbnailsActivity/src/image/Thumbnailsline36
Java Problem
R cannot be resolved MediaPlayerDemo.java /Media/src/media/media line49 Java Problem
R cannot be resolved MediaPlayerDemo.java /Media/src/media/media line50 Java Problem
R cannot be resolved MediaPlayerDemo.java /Media/src/media/media line52 Java Problem
R cannot be resolved MediaPlayerDemo.java /Media/src/media/media line56 Java Problem
R cannot be resolved MediaPlayerDemo_Audio.java /Media/src/media/media line80    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line106    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line107    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line112    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line114    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line116    
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line118 
Java Problem
R cannot be resolved MediaPlayerDemo_Video.java /Media/src/media/media line120 
Java Problem
R cannot be resolved videothumb.java /Media/src/media/media    line 26    Java Problem
R cannot be resolved videothumb.java /Media/src/media/media    line 41    Java Problem

Something is definitively going wrong her开发者_如何学Ce.

Can you help me? I am stuck...

new for android application....

In eclipse on the left side , in the package explorer right click on the project, then go on Build Path , then select configure build path ..

PojectName--> BuildPath --> configure build path

You will see a window with title Properties for ProjectName

in this window a list tree is drawn in a white panel, in this tree select Java Build Path... after this you will see a tabbed pane with 4 tab , select Order and Export tab..

You will see a checkbox item with your target sdk name,, check this and refresh...

This should work.. if not then clean the project and rebuild again...

To clean go to the project menu of eclipse select clean and proceed further