A diode is an electronic device that acts like a one-way valve, letting current flow in only one direction through it. The diode has a maximum current and voltage rating, so these two parameters will strongly determine the one you select. Your circuit may destroy an underrated diode, and an overrated diode may be too big or expensive for what you need, so you'll need to use some care in picking one. Additionally, you'll want to choose a diode packaged the way you want to use it, such as surface-mounted or with leads.
Examine the schematic and determine the maximum current you expect will flow through the part of the circuit that has the diode. Determine the maximum voltage difference the circuit will produce between the diode's two leads. For example, if the potential on the diode's anode side reaches +100 volts and the cathode side goes to -25 volts, the total difference is +100 -- (-25), or 125 volts.
Multiply the voltage and current figures by 1.3 and write them down. These are the minimum ratings that will suit your application. Now multiply the original voltage and current by 2.5 and write these figures down. This are the maximum ratings your circuit should need.
Open the catalog and look for diodes that fit between the low and high current limits you calculated in Step 2. If the diode is relatively large and comes in a bolt-on type case, make sure the physical design of your circuit will accommodate it. Likewise, consider other physical packaging issues for the device, such as if you need a diode with leads or a diode in a surface-mount package (SMD). Of the diodes that match your packaging and current requirements, find one with a peak inverse voltage (PIV) or peak reverse voltage (PRV) rating that fits within the low and high voltage limits you calculated. If you exceed this rating you will destroy the part.