Summits On the Air
Summits on the Air (SOTA) is an activity where Activators go to a summit and attempt to complete at least four contacts on amateur radio bands. Summits are worth varying numbers of points, depending on the altitude above sea level. Chasers can also get points by contacting people who are on summits from their home station, or from another summit (which is called Summit to Summit or S2S).
To activate, you must hike up an approved summit. You must operate from within the Activation Zone, which is an area within 25 vertical meters of the highest point. So if you look at a 1000 meter summit on a topographical map, you can activate anywhere inside a contour line around the summit at 975 meters, which may cover a very large area if it is not too steep. You must carry all of your equipment with you to the activation zone, and operate on any of the legal amateur bands in your country. You must log your contacts on the SOTA database: https://www.sotadata.org.uk/en
Be safe. Follow the "Leave No Trace" principles. Carefully plan a hike which is within your own ability and skill level, and let someone know where you are going and when you expect to return.
The best source of information about Summits on the Air is https://www.sota.org.uk/. Don't be fooled by the domain, this website is not specific to the UK, it's the official site for SOTA globally.
The official site has a map of its own, but it is not very user-friendly. The easiest online mapping tool for finding summits is https://sotl.as/map.
Self-spotting is allowed and recommended. Spot yourself on https://sotawatch.sota.org.uk/en/ and chasers who monitor the spotting reflector will seek you out, making it much easier to activate.
Log your contacts on https://www.sotadata.org.uk/en
Official guidelines for activators can be found here: https://www.sota.org.uk/Joining-In/Guidelines
Specific rules for your area can be found by finding your Association in the list linked below. Search for your country, and click the appropriate Association. In the US, it would be W2 if you're in the 2-land call area. Then click Association Reference Manual: https://www.sotadata.org.uk/en/associations
You can activate a summit with just a VHF Handheld, especially if you are within line-of-sight of a populated area. An antenna with more gain than a rubber duck is recommended (such as an Arrow or homemade yagi) and you may benefit from a bandpass filter if the summit is home to a commercial transmitter. Most contacts are made with a light weight QRP rig using CW.