Insulating your loft is important as you could be losing up to 1/3 of your heating cost through your roof.
You may be surprised to know that the current recommended depth of insulation is 270mm (10.5 inches). This is quite a thick layer!!! You may find you already have some insulation in the loft. If it hasn't been done for many years it will probably be quite thin.
NB some councils will give grants for loft insulation. Check it out you may be able to save some of the cost!

There are various types of insulation available. For a DIY er the choice is between a:
Mineral wool Quilt eg Rockwool sold in rolls in big bags of varying width and thickness.

Loose fill . Sold in bags of granules and poured between the joists.

Given that the current recommended thickness is 270mm or 10.5 inches I would recommend using Mineral wool. You should find that this is readily available at your local builders merchants or big DIY stores.

This is a fairly easy job although it will be a bit dirty and dusty in the your loft. Make sure you use a dust mask and gloves. You may find mineral wool will itch if you touch it too much without gloves.

NB. You can only stand on the loft joists. Standing between them is dangerous as this will not support your weight. Use some sturdy wooden sheets or planks across the joists as a temporary platform .

You'll need a couple of layers to achieve 270mm. I like to lay the first layer between the joists. Then lay the second layer on top across the joists.
If your roof space has gaps at the eaves they will be for ventilation. Don't block these with the insulation or you may get condensation forming.
Don't cover any light fittings with insulation (like down lighters) as they need space to dissipate heat.

Don't cover any house wiring either. If in doubt get an electrician in.