In the depot you are able to buy, sell and schedule all available vehicles of a particular type, i.e. road vehicles if you're in a road depot.
The depot dialog contains information about the vehicles you are able to buy, and those which you already have bought. In the tabs at the right the buyable vehicles are displayed. In the area top left the currently assembled convoi (i.e. a train) is displayd. In the middle left area engines and trucks are displayed and in the lower left area waggons and trailers are displyed
At the lower border there is a information area that displays an overview of the currently selected vehicle, and below this there are three buttons.
Leftmost is the 'start' button, which will allow you to set off the scheduled convoi, the 'schedule' button which will allow you to schedule the currently assembled convoi, and the 'sell mode' toggle which will allow you to sell vehicles.
To see the information of a particular vehicle hover over it's corresponding image. This will show all available information in the lower info box of the dialog. To a buy a vehicle left-click it from the tabs at the right. It will appear to the left of the dialog box. The convoi that you are currently working with will appear in the top box, with any additional engines/trucks/ships appearing in the box just below this and any additional carriages/trailers appearing in the lower middle box, above the info box.
To create a convoi you can either buy all needed vehicles one after the other, starting with the engine/truck and then buy any needed cariages/trailers. Or you can select any vehicle from the additional boxes and make a convoi from them. Only one vehicle can be worked on at any one time. To sell any of your brought vehicles select the 'sell mode' toggle button, and then with this selection hover over any of your vehicles and click on it. The vehicles selling value is shown down in the info box at the bottom.
To schedule a vehicle click on the 'schedule' button to bring up the schedule window. Once you have scheduled a vehicle you can start it simply by clicking on the "start" button.
To send a convoi back to a depot later on, remove all stops from the convois schedule and include the depot location as waypoint. Once the convoi reaches the depot it gets disassembled and the vehicles become available in the depot again. You can then sell or rearrange them.