In order to preserve anything properly, you'll need to sterilise your jars and their lids or you'll risk your preserves going bad at best, and botulism at worst.
There a number of ways that jars can be sterilised:
Water Bathing (this is my preferred method, mostly because this is what my mother always did when making cordials and jellies and it has always yielded good results)
Take your jars and lids, wash them thoroughly in hot, soapy water removing any labels and paying close attention to the rims and edges of the lids. Immerse them in a deep pan of boiling water and simmer for ten minutes. Carefully remove them from the water with a pair of tongs. Fill the jars to within a centimetre of the top while still hot and put the lids on.
The Oven Method (I've tried this a few times and have found that jars tend to break, possibly due to the much higher temperatures reached in an oven than in boiling water)
Preheat your oven to 140°C / 275°F / Gas Mark 1.
Thoroughly wash your jars and lids in hot, soapy water and rinse well. While still wet, place your jars on a baking tray and put in the oven for 20 minutes. Make sure the jars are not touching each other.
Meanwhile, in a small pan, boil the jar lids for at least 10 minutes.
Fill the jars to within a centimetre of the top while still hot and put the lids on.
Microwave (great for when you only want to sterilise a few jars!)
Wash your jars and lids thoroughly in hot, soapy water and remove any labels. Rinse them well. Put the jar in the microwave on full power for about 45 seconds, or until dry. Boil the lids in a small pan for 10 minutes. Fill jars to within a centimetre of the top while still hot and put the lids on.
Dishwasher
Wash jars and lids in the dishwasher on maximum temperature for a full wash and dry cycle. Fill the jars to within a centimetre of the top while still hot and put the lids on.
Sealing lids:
If the contents of the jar is hot, as it cools down it will create a vaccuum inside the jar and the lids will seal. If the lid doesn't fit properly or has been damaged, it won't seal properly so always check before storing your preserves. This is done by pressing the 'button' in the centre of the lid. If it bounces back and 'pops', your jar has not sealed. If it doesn't bounce then the jar has sealed successfully. When opening a jar of preserves you should listen out for the 'pop' as you break the seal. If this doesn't happen there may have been a problem with the lid and the seal and your preserve may not be safe to eat. Remember, botulism has no smell and no taste so it's always better to err on the side of caution and discard any preserves you are not sure about.
Simple rules:
Always put hot contents into hot jars! This not only prevents the growth of potentially dangerous bacteria, it also stops the jars from cracking.
Fill the jars to 1 cm from the top. Again, this helps to prevent the growth of bacteria which at best will spoil your preserves and at worst could cause botulism.
Store in a cool, dry, dark place.
Check the seals before storing and when opening! If the jar hasn't sealed, don't store it; if seal is broken when you come to open a jar, discard the contents.
Use your senses! If anything looks or smells or tastes wrong when opening a jar, discard the contents.