There are many different accessories available to buy for your model railway.
These can range from essential things such as scenery for your layout, to more complicated things such as working signalling equipment.
Many modellers choose to make their own accessories for their layout - as there is often not a suitable commercial product available, others buy everything and will only need to put everything together and add it to their layout.
Most major manufacturers of model railway locomotives and rolling stock (Hornby, Bachmann etc) also offer a range of accessories to compliment their products.
The type of accessories you will require depends on the scale you are modelling in. You'll almost certainly need some form of track pins to hold your track pieces in your baseboard.
Another type of model railway accessory is model buildings. If you are able to photograph a real building to base your model on, these can be made very easily, as you can simply stick the photographs to a cardboard "shell" and instantly have a replica of that building.
Kits are also available for railway buildings, these usually need some basic assembly, which anyone should be able to do with very little trouble.