what can i do withthe ds can i play any hombrew or what are my otpions of doing?

There are many things you can do. There is a lot of homebrew out there. Are you looking for games, applications, backup capabilities? Ask away, and we will try to answer for you.

Regardless of what homebrew you want, you need a cartridge.
There are 2 types:
Slot-1 and Slot-2

Slot-1 goes into the DS slot, and is very easy to use, but cannot play GBA backups.
Slot-2 goes into the GBA slot and is a little more complicated to setup, but it can play GBA backups.

which cartridge? and how do i do it?

Answer my questions 1st then I can help you more.

im looking to do homebrew!

Currently homebrew compatibility is not that good with most of these cards. However it is improving with firmware updates.

Slot 2 has the best compatibility for homebrew right now. That will change in the future. So, if all you want is homebrew, get the cheapest slot-2 cartridge you can find. Remember to add the cost of the passthru card.

For homebrew of the future, get a slot-1 card. The DS-Xtreme has the best compatibility of the slot-1 cards but limited space 512 MB.

Do you want homebrew for movies and music? If so, then you might want to get the M3 Simply or the R4 instead. They have expandable memory.

If you would just give some details, instead of acting like a jerk, maybe you would get more help, and faster. What kind of homebrew? Like I asked a week ago.

looking to do play emulators ,music ,and videos . ?