Attenuation will reduce overall signal level without changing the receiver noise floor, preventing the reception of very small signals. Generally you do not need one unless you are connecting the input directly to the output of an RF source with no antenna, or your antenna is very close to a powerful transmitter antenna. -5 dBm is very 'hot', generally you're going to be dealing with signals much weaker than this. For example, my computer reports the wifi signal strength from a router in the same room as -46 dBm. Signals from far-away sources can be much lower, for example GPS is around -160 dBm.
External attenuators can be simple RF devices with a male SMA on one end and a female on the other end, so you can just screw it onto the input SMA connector and then attach your SMA cable on the other end of the attenuator.
Attenuators are rated both in dB and in watts. The dB rating is how much the attenuator shrinks the signal passing through it. A 10 dB attenuator will drop a -5 dBm signal down to -15 dBm. The watt rating is a thermal rating for how much power you can safely pass through the attenuator. Many attenuators can handle a watt (30 dBm) or more without difficulty. If you need to look at very powerful signals, it may a better idea to use a directional coupler and a load. Attenuators are also specified in terms of frequency range, usually DC up to some maximum frequency.
If you want to look at a -5 dBm signal, I would recommend using at least a 10 dB attenuator. It's a nice round number that is easy to account for later if you want to know what the power level was at the input of the attenuator, and it provides protection if your signal exceeds -5 dBm by a little bit. It might be worth getting a larger attenuator of 20 to 30 dB or possibly getting a pair of attenuators for more flexibility - e.g. 10 dB and 20 dB, use both in series for 30 dB.
