Jam is dead easy (I made a batch once when I was so concussed I couldn't see straight). You need a big pan, and if it has a heavy bottom it means less stirring! You also need a clean (ie not garlicky) wooden spoon, that's long enough for you to stir the bottom of the pan without sticking your hand too near the boiling sugar.
Equal weights fruit and sugar in your pan (less sugar is possible, but won't keep for more than a few months). If you can, adding some under-ripe berries, or a finely chopped Bramley, will help the set (runny jam isn't the end of the world, but doesn't keep so well). Start to heat it gently, so that the juices run and the sugar dissolves. Once it's all dissolved, turn the heat up to a fast boil, and keep stirring. The jam will try to boil over - find the point just below that, and if it does get away from you, blow hard on the foam, stir like crazy, and pull it off the heat! After a few minutes, you'll notice the texture of the foam starts to change (the bubbles go sort of glassy). Keep it going, and make sure it doesn't stick.
There are several ways to test to see if the jam's set yet. I find the best is to get a cold saucer and dribble some of the mix on it. Let it cool for a few seconds, then push it with your fingernail - if the jam's boiled enough it'll form a skin and be gooey rather than syrupy. If it's not ready, put the pan back on the boil, and wash the saucer in icy water (dry it off well!) so you're ready to try again. It is possible to overboil jam (you end up with a sort of fruit-flavoured toffee). If it starts to caramelise, you've gone too far - take it off the heat at once! Underboiled jam is much more useful than overboiled - it's a wonderful ice-cream syrup for starters.
Once your jam's ready, you need to pot it. If you want to keep it for any length of time, it needs to be sealed in sterile jars. The easiest way (far easier than faffing around with paper and cellophane) is to save jam-jars with undamaged lids. Wash them well, then either boil them in a saucepan (best for a few jars), or fill them with boiling water and put them in a warm oven (best for a large batch). Scald the lids too. Being very careful not to pour boiling water on your oven gloves, get a jar out, tip the water away, fill it with boiling jam (easiest with a jam funnel; if you don't have one, work on a baking sheet so the spilt jam isn't wasted), and screw the lid on firmly. Repeat. As the jars cool, the dimples in the lids should pop down (if one doesn't your seal isn't good enough or it was too cold when you filled it - that jar won't keep so long as the rest). If there's a partially filled jar, finish it first (after the pan scrapings and any you spilled!).