There are several factors in choosing the right substrate. The main factor is the humidity. If your reptile requires a high humidity then you want to use a substrate that can hold moisture well. A few examples are Orchid Bark, Sphagnum Moss, Coconut Bedding and Komodo's Tropical Terrain.
There are a few other options for reptiles that do not require high levels of humidity, each with their own advantages and disadvantages. Beech chips are one of the most popular though others include Bio Grass, Repti Maize, and Reptile Carpet.
The main problem with Beech chips (and indeed any substrate that is in particle form) is that there is a risk your reptile could accidentally swallow it while eating which can block the digestive system or cause serious internal damage. A safer alternative is Bio Grass which is basically compressed grass pellets. These pellets break down when wet and so can be easily passed through if swallowed. This is also true to a lesser extent with Repti Maize which is made out of kiln dried corn. When swallowed Repti Maize goes mushy around the outside and so is not so hard to pass through and less likely to cause internal damage like with beech chips. The main problem with using these kind of substrates is that they will disintegrate if they get wet which can be a nuisance around their water bowl. Of course all ingestion problems can be avoided if you use a substrate that isn't in particle form like Reptile Carpet which is a coarse felt like material you can use to line the bottom of the vivarium. This will eliminate the ingestion problem, though it will prevent lizards who like to dig into the substrate from doing so.
In some cases it may be more appropriate to feed your reptile outside of the vivarium to avoid the risk of ingestion.