well the topic and the question your asking sounds different. do you want to know how to play games online as in wifi? or how to play games you got from the online? to play wifi usually games have an option that says wifi. or something like that and you just connect to a hotspot or router. to play games or roms youve downloaded, you either have two options. to get a flashcard for your ds (notably r4,m3,g6). and after getting that, just follow some directions and you should be playing roms in no time. OR find an emulator to download somewhere online, and just google it or something and such to use the emulator.
What? All I was trying to say is: can i play roms on the internet, roms that i have downloaded online. I have all the stuff and i can play roms on single player mode. Is my English really that bad?
Read Dragnandys post, you need a wireless network, a flashcard, a MicroSD card and a DS, I don't know of any emulators compatible with Wifi. This has been spelled out to you, the only reason I doubt your comprehension of English is that you were unable to read it.
I know the snes emulator Zsnes has a great network/internet multiplay option. As long as both of you have the same version of the emulator, and the rom, one of you can run in server mode, give the other player your IP address, and they can connect and play with you. I used to play Super Mario All Stars with a girl online all the time. Its really not that hard. now, as for a community where you can just sit around on a saturday night and say "Hey, I miss the days of the 90s. Id love to go down memory lane and play some old games online!" as far as I know, there is no generic way of searching for peers to play with. But I have ALWAYS thought, since I first discovered emulation well over 10 years ago, that someone should creat some kind of simple client/server based program for us to find people to play roms with. I know i only used "zsnes" being an snes emulator as an example, but I have seen internet/network multiplayer options on some Game Boy Advaced emulators as well.