In order to fight a battle, you will need somewhere to set up your battlefield - a flat, stable area unlikely to be ravaged by natural disasters like The Feet of Doom or the equally terrifying Household Pet Monster. Some people make do with a smooth bit of floor (although this is likely to fall prey to The Feet), but most use a largish kitchen or dining table (suitably protected by a cloth or blanket). By far the best option, if it's available, is to fight over a gaming board made up of sheets of chipboard, plywood or MDF laid over another table. Typically, the battlefield should be between 6 to 8 feet long (around 1.8 metres to 2.4 metres) and 4 to 6 feet broad (1.2 to 1.8 metres). You can play on a smaller area quite easily, but you'll need to keep the forces fighting proportionately smaller to ensure that you've got some room to manoeuvre.
Building up a collection of scenery is an important and enjoyable part of the hobby. It is possible to buy all kinds of terrain like trees, bushes, hedges, fences, ruins, tank traps, crates, barrels and card or plastic buildings. Take a look in your local Games Workshop store (if you have one nearby) and you'll find all kinds of scenery that you can buy as well as textured cloths for covering your tabletop and lots of other useful tools and materials. Failing that, model shops and particularly places that specialise in model railways have a good selection of scenery, although it is often comparatively expensive.