Soldering is a physical skill, and like any physical skill it is easier to learn properly once than to unlearn later. These five habits cover most of what separates a reliable joint from a frustrating one.
1. Heat the joint, not the solder
Touch the iron to the pad and the component lead together, wait a second or two, then feed solder into the joint, not onto the tip. Take the solder away first, then the iron. The whole thing takes three or four seconds.
2. Keep the tip clean and tinned
A dull, oxidised tip will not pass heat. Wipe it on brass wool or a damp sponge and melt a little fresh solder onto it every few joints, and always before you put the iron down.
3. Use the right solder
Thin rosin-core electronics solder, around 0.8 mm, is what you want. Never use plumbing solder or acid flux on a circuit; the residue eats copper.
4. Learn what a good joint looks like
A good joint is a small smooth cone that has flowed onto both the pad and the lead. A ball sitting on top of the pad, or a grey, grainy lump, means one of the two surfaces never got hot enough. Reheat it and add a touch of solder.
5. Look after yourself
Wear safety glasses, because molten solder can flick. Keep the smoke out of your face with a fan or an open window. Return the iron to its stand every time, and wash your hands when you finish.
Practise on a scrap board first. Twenty joints in, your hands will have worked it out.
If you want the fundamentals in one place, with hand-drawn diagrams, start with Getting Started in Electronics, or browse all four books.