Add electrical generator breaker to service box
You are not going to go very far with trying to add the things that you desire to that electrical service box. It is already completely full of duplex circuit breakers AND there is no main breaker in there to interlock with the generator breaker.

You will need to replace that box with a new larger box that is equipped with a proper main breaker and is compatible with a proper agency registered generator transfer switch. 

Some things of note:


That generator inlet box looks like it is setup with some pretty small feed wires. You should think very carefully how you should really setup the limited capacity of the generator to feed only a subset of breakers and possibly relocate those to a sub panel.
There is a very sketchy use of a black taped white wire being fed from a  breaker. If at all possible you should be looking at replacing that wire run with a proper colored wire.
The wires jumpered into the lugs where the feed wires are terminated is an extreme hazard waiting to happen. If one of those wires were to get shorted somehow it would not be able to handle the current that those feed wires can deliver and will soon get extremely hot and cause a fire and melting wires and insulation. Disconnect those immediately till that can be properly corrected.
It is next to impossible to decide if this service box is a main-panel or a sub-panel. Along with that it is not possible to see if the ground / neutral bonding is properly handled in this enclosure. It would be wise to get that checked and brought up to code if needed.
With no visible bare safety ground wires it is clear that the grounding system being used here depends upon the conduits that carry all the wiring to this service panel. You should double check that every last outlet, switch and lighting electrical box is properly terminated to metallic conduit all the way back to this service box to ensure that the intended safety goals are met.