When I started designing games I was completely lost as to how to get resources and create sprites and sound effects so over the years I have found many useful tools that should be in every game designers tool kit and these are my favorites. If you have others not on the list send me a PM and I will add them.
This is a free paint program that has the ability to edit sprites if set as your default outside editor in C2. I like it because you can use the magic wand to remove stuff and rotate a sprite and set the sprites to different effects.
This is a free sound and music editor that also records and handles many file types. I use this to edit sound effects and to convert MP3 and Wave files to OGG files for C2. Many plugins available and easy to learn while having many advanced features available.
This is a free neat little program that allows you to make stickman figures or any figure and change positions of the figure using rotating joints. Allows the figure to be posed and each pose saved as a separate picture file to be imported into the C2 editor.
Good free program for creating 2D sprites.
This is an online Sprite Editor.
This is an online program to create character sprites with different features. Lots of good animations for each character.
Online character generator with lots of features and in multiple languages. Many characters created by outside designers.
A good place to find unique art and sprites for your creations. Be aware some may be copyrighted and always give credit and ask permission.
The Spriters Resource
Excellent for finding sprites used in professional games on all platforms. Be aware these are copyrighted but with modifications or for your own use they work great.
A free resource of sprites, sound effects and art for Game designers.
You can find just about anything using google search if you use the correct search terms:
Add CLIPART to your search and you will find tons of free pics that can be used in your creations.
Add SPRITE SHEET to your search and you will find tons of ready made sprite sheets.
Add TILESETS to your search and you find tons of ready made tilesets for your games.
This is a free drafting program I use a lot for making 2D objects in perspective so they appear to be 3D. It is a more advanced tool but something any designer should have and there are tons of designs in the Google Sketchup Library and you can get addons to do Rendering.
This may be on your computer already or you can download it from the link. This is great for capturing images from your screen and I use it every day.
Free game sound effects and many are public domain.
Most of these sound effects are copyrighted but can be purchased for a small amount or used in your non-commercial games. This is a huge library.
The Freesound Project
No Soap Radio - Free music organized for game genres.
Jamendo.com - Modern indie music released under Creative Commons
The Video Game Music Archive - Music from old video games. There may be copyright issues if you widely spread or sell your games with this content.
Public Domain 4 U - Old and new music that is in the public domain.
SimplytheBest Sounds - This site has both music and sound effects.
OK that should be enough to get you started on your game designs.