Your processor will send data to your radio transmitter chip. Radio transmitter chips can use the amateur (HAM) bands, such as the IOS-recommended Radiometrix TR2M, or the Industrial, Scientific, and Medical (ISM) band (with a Microhard n920 or Microhard n2420). Your transmission strength is likely limited to half a watt—but what to send? The answer is packet data.
Packet data is a way to send digital information (such as image data or sensor readings) using analog radio. Your ground station will need to be able to handle packet data, directly interfacing with a computer. Your satellite will simply connect your Arduino or BasicX-24 to the transmitter chip.
Transmitter chips are smart; if you send them the appropriate-sized packet with necessary headers, they transmit. Therefore, as a minimum, you will need to bundle your data into a data packet containing your ID, a timestamp of when the data was taken, any sensor ID required that lets you know which instrument was used, and the actual numeric value of the data. You need to process all incoming data and decide which to transmit. Amateur HAM uses the AX.25 data link layer specification to define its packets; however, within that specification, you can use any underlying data representation you wish.
Technically, you can maximize your effective bandwidth by ignoring data timestamps and broadcasting just data values, with the time then being assumed to be sequential and close to the time the data was received. This would be ...