To pick aquarium decorations, I recommend you go to a local store and get the advice from somebody who works there. There are a lot of beautiful decorations that you can pick for your tank and there's also a lot of things that can be potentially be dangerous for your tank. What I do recommend or actually don't recommend is that you find rocks, stones and pieces of wood from outside, that look great, that you want to put in your tank or lets say you go to the beach and you grab some coral or bottles. Some of those things can leach chemicals, stuff that can hurt your tank, that can throw the pH off in your aquarium. So you really want to stick with aquarium decorations that are suitable for your aquarium. The best place to get that, is at your local fish store or online.
Once you've got a selection in front of you, you want to choose from plants, rocks. Things that are inert and won't react with the water, unless it is your goal to have the decorations help buffer the water chemistry. In certain situations, like for African cichlids, people buy decorations that purposely release carbonates into the water to help buffer the pH of the water. In general, you want to pick decorations that are neutral. Plastic plants work great. You buy them with the little plastic anchors that you can fill with gravel. That will help keep them weighted down. Some fish like to kick up the plants and redo the tank decorations on their own. So you'll find that the heavier the decorations the better.
And just build some caves. You want have a lot of water flow going in and out of the rocks and the decorations so that a lot of the waste can get carried through the water column and get picked up by the filtration. The fish also need places to hide and they're very territorial. So the more decorations you have in the tank, within reason, the happier the fish will be. A very sparsely decorated tank will result in some fish feeling very territorial and chasing the other tank mates and keeping them up in a corner.