The New York Times reports  that some aid has begun flowing into Myanmar, but it looks like the mobilization for major relief operations is still underway and not clearly defined. However, you can keep track of what's going on in this regard by visiting the site that ReliefWeb has set up  for the disaster caused by cyclone Nargis in Myanmar.
ReliefWeb , administered by the UN Office for the Coordination of Humanitarian Affairs (OCHA), is the main gateway to information --including news, documents and maps-- on humanitarian emergencies and disasters. It's targeted to the international humanitarian community that works on delivering emergency assistance, so you can hardly find a better site for up-to-date, reliable information on all aspects of this emergency.
Incidentally, there's also a site on the effects of the floods caused by Nargis in neighboring Thailand .
As for individual donations, it still seems too early to know how or where to contribute. I'll try to post this information later as it becomes available.