Spreading mulch is one of the best things you can do for your garden, as it adds nutrients from decayed plants back into the soil: like in nature, but better!
What is Mulch?
In the broadest sense, mulch is any substance that you spread over soil in order to keep the sun off it while still allowing rainwater through, thus preserving moisture and suppressing weeds.
It also makes pulling up weeds easier, because the weeds that do come through will tend to root in the loose mulch more than the soil, at least when they are still young.
Organic matter is the best mulch because it slowly decays and feeds the soil, but inorganic stuff like gravel works too.
In the ornamental garden, most people prefer aesthetically pleasing substances like bark chips, which come in pretty colours and last for a long time.
In the fruit and vegetable kitchen garden, well rotted mulches that release nutrients quickly are preferred, and these are typically topped up every year.
Straw is also common, being so widely available, although it won't add much to the soil nutrient-wise.
Any organic matter that does not contain weed seeds or persistent herbicides makes decent mulch.
You do not dig mulch into the soil, so it is not a growing medium like well rotted compost.
Mulch supports the soil below it, which is where the roots of your plants are (some rugged plants will root anywhere, of course)
Yes, well-rotted material will release nutrients immediately, especially composted manure, so it is better for hungry plants, but un-decayed stuff is fine: that is how all mulch in nature starts off!
The big exception is fresh manure from animals that don't ruminate, like horses and birds, which is often too high in nitrogen (known as "too hot") for your border plants (mature trees and large shrubs with deep roots should be fine).
Their manure should be well rotted before spreading around smaller plants, especially vegetables.
Cow, goat, and sheep manure is double-digested, and does not have this problem.
Woodchips make great mulch in most situations, other than your vegetable beds.
There are two old chestnut myths about using woodchips,
Myth 1: Fresh woodchips rob the soil of nutrients.
That would happen temporarily if you mixed the fresh woodchips into the soil, but mulch sits on top of the soil like a moist, nutritious blanket.
Myth 2: Woodchips from conifers (like your old Christmas tree) acidify the soil.
It depends on what pH soil you start with.
If you have high pH, alkaline, chalky or limestone soil, and cover it in organic matter, you will acidify it in the sense of lowering its high pH down towards neutral. But not in the sense of making it acidic, or low pH.
If your soil is, like most gardens with normal topsoil, already neutral or acidic, mulch will not have a significant effect on soil pH.
You would need to use a soil acidifier for that, typically Sulphur, or fast acting Aluminium sulphate for hydrangeas.
You can use conifer woodchips anywhere you like, it won't make the soil acidic.
How and When to Apply Mulch
The best time to mulch your garden is late winter / early spring, when the soil temperature is rising. But as with so many things in the garden (and life!), the most important thing is not when you mulch, but that you mulch.
The only potentially bad time to mulch is when the soil is parched dry in summer, because the mulch will prevent light rains from penetrating the soil.
During summer, always mulch after heavy rains, or a thorough watering.
You will need some organic matter like compost. and a garden fork or spade.
The inimitable Robert Pavlis lives in Ontario, Canada with a similar-ish climate to the UK, and describes your main options