If you are planning to have bottom dwelling fish such as Pleco's, they need wood in their diet to help digestion
You can purchase wood from you Local Pet Store which can get pricey if you want a large piece, or you can find it in a river or beach. Preferably the beach, there you will find sunken wood such as large twisted roots which can create some cool looking effects in your tank, make sure that you use a hardwood where possible it will last longer in your tank, and have a much nicer colour. Typically sunken wood from the beach will have a high salt content and is recommended that you soak it for up to a week, the benefits of salted wood is you know that there wont be any freshwater nasties unlike wood from rivers, It can be a little risky, as freshwater diseases can be carried on the wood, make sure that you clean it thoroughly, river wood is generally younger and can leach out tannins making the water go brown, but don't fret.. the fish will love you for it. To avoid this brown water, you could soak the wood for a few days/weeks to your liking.
What about Ornaments?
Perhaps you are going for a themed tank, like the one below. Its up to you really, there are no benefits or drawbacks with having plastic ornaments, they are purely cosmetic. Although they may need a clean one every once and a while.
Rocks are great, just be careful not to get rocks that have any metal deposits in them such as copper etc.
