The aim of mulching is twofold: to retain water in the soil by reducing evaporation and reduce weed growth by excluding light. You can mulch with a variety of materials including stones & landscape fabric, but the best is well rotted organic matter as this brings a 3rd benefit: over time worms will drag it down into the root-zone and enrich the soil structure and nutrients. ‘Organic matter’ includes, garden compost, leaf mould, composted manure or anything well-rotted that once was a plant. Making your own compost is very satisfying but takes time and most people cannot make enough for their whole garden at least for the first few years. Some local authorities collect householders green waste and compost it at high temperatures so that weed seeds and roots are killed off. You then buy it back from the recycling site for use as a mulch: a bit like outsourcing your compost heap! Chipped wood or bark also makes a good mulch that lasts longer and is clean and decorative. Fresh chippings do rot over time and if there is no green leafy content they can temporarily take nitrogen from the soil to decompose. To avoid this, I usually use chippings over landscape fabric for paths and shrubberies or over a layer of garden compost in flower beds.
Happy Gardening, Alison