Before you create a topic asking how to modify GTA San Andreas, read the SA Modding Guide.
For more specific help, see the List of Tutorials. If you have version 2 of GTA SA, you need to Downgrade. You can also read about PS2/Xbox Modding. If you want to suggest a mod to be created, post that in the Mod Request Thread.
Use IMG Tool for editing IMG files, and TXD Workshop.