A Frequently Asked Question:
WHAT DO I NEED TO DO TO START CREATING OBJECTS FOR THE SIMS?
"Object Making" is the term used for altering the look of objects.
"Hacking" is the term used for altering the object's function.
Either way you will need a cloning program.
With this program you "clone" your own copy of the objects.
Mac users use IffSnooper
Mac or PC user, be sure to pick up (or make) a "Magic Cookie" number before your start. This is a number you will enter into Tmog (only once and it remembers the number for next time you want to make objects) and needs to be a number that no one else uses. Tmog will use this number to figure out what Object ID numbers to give all of your objects (and those must all be unique to prevent Object ID conflicts)
For Object Making you will also need a graphics program. Which you choose is entirely up to you. The most popular (especially when it comes to online tutorials) are JASC's Paint Shop Pro and Adobe's Photoshop. There's also a free program around that I hear is excellent called GIMP.
Google (or other search engines) can help you locate online information about these three paint programs.
There's a series of fantastic tutorials on making objects at The Bunny Wuffle's School of Sims Transmogrification http://www.strategyplanet.com/thesims/sas/bwsost/bwsost.htm
The program used there is Photoshop so if you use another program you will need to be familiar enough with it to 'translate' her instructions. The best part about these tutorials, however, is that they teach how the sprites actually work. This invaluable information applies no matter what graphics program you are using to work on the sprites.
EDIT: Simblesse Oblige now has a sort of translation dictionary for graphics programs that you can use when folloing a tutorial that uses a graphics program you do not use. Simply look up the tool mentined in the tutorial and see what the equivalent tool is in your own graphics program. Entries have been made for Photoshop, Paint Shop Pro and GIMP.
To Hack objects you need an iff editing program.
PC users have a few choices but the most common, most complete one is IffPencil2. (the "2" is now sometimes dropped so it's just called IffPencil and occasionally IP2 or IFF2)
Hacking tutorials were always a lot more scattered than object making tutorials. Most of the information was found in a few key sites and forums. As hackers moved on to non-sims activities and to The Sims2 these forums and sites dwindled away and domains were not renewed.
One that remains is Sims College. No longer given in the form of classes, this site has left it's material up as online tutorials for anyone who wishes to come learn it on their own. Unfortunately many people are having problems accessing the classroom now. For those who can access the material this is the best place to start learning to hack.
Those having trouble with the original site can access the material mirrored here at the Sims College Hacking Lessons (Archived)
The PC program IffPencil is used for these lessons so mac users will have to translate the material into IffSnooperese. As most tutorials available are written for IffPencil this is something you will need to do a lot and The creator of IffSnooper has written a tutorial to help with this:
Additional "How To Hack" information is available at the Woobsha Workshop
TIPS AND TUTORIALS - THE NEXT STEP
There are also tutorials around that address specific hacking and object making projects. Rather than list them here I will simply refer you to Quirk Sims's wonderful "How-To's" page: http://www.quirksims.com/misc/howto.html
An excellent resource for creators of all types and skill levels has opened, Simblesse Oblige. It is a (free) forum allowing you to communicate directly with other creators, getting help, sharing ideas and helping others along.