The easiest way to start your first aid kit is to buy a ready made one.
Having done that however, check the contents and supplement them to suit your own needs and in particular the needs of someone on a long distance trek.
So consider also taking corn pads to protect blisters, blister dressings such as 'Second Skin' or 'Compeed', additional plasters, a needle to drain blisters, and painkillers in case you get a headache.
And if you're not first aid trained it's also worth obtaining a book on first aid , there are some excellent ones available.