Slayer is regarded as one of the most profitable skills in runescape. Some monsters like Aberrant spectres, Abyssal demons, Dark beasts, and Gargoyles drop many things that you can sell for a good amount of gp. But these require certain combat levels. So if you are low level, you can start from cooking trout, cutting oak trees, or making iron bars, because these are low requirement money making methods. This guide should help you to make some money if you are low level.