"Subject" I want to start making content for them... but don't know where to start. Someone? THanks

The usual ones (jpg, gif, png, etc.). And different cellphones will support different formats differently. So you would need to check the manufacturer's site to see what is listed for different models.

What kind of content are you planning on working on? Because if it's graphics for games then the platforms tend to be a bit more restrictive, and often even if a cellphone supports a file format (so you can see it in the gallery, or set it as a wallpaper) it won't support that same format in it's programming platform. For example, J2ME only guarantees support for png, all other formats are optional. BREW apps use a (slightly different?) bitmap format if I'm not mistaken.

shmoove

mmmh, interesting. I'll check the sites of Nokia and Motorola.

Thanks for the help.

Regards. Rodrigo Contreras