All trekkers are required to register their trek by obtaining a Trekking Information Management System (TIMS; www.timsnepal.com) card. The card costs the equivalent of US$20 for individual trekkers or US$10 if you are part of a group. SAARC trekkers pay US$6 (US$3 for groups). You need to show the TIMS card at the start of the Annapurna and Langtang treks.
At the time of research, the local authorities in the Khumbu had introduced a Rs 2000 'entry fee' to the Everest region, doing away with the need for a TIMS card, but this could change, so check before heading out to Lukla.
The place to get a TIMS card in Kathmandu is from the Tourist Service Centre, mainly because you can also get conservation-area and national park tickets in this building. Bring your passport and two passport photos (though you can get free digital photos here). The card is issued on the spot; green for individuals and blue for group trekkers.