For most small DIY jobs I find a 20 inch general purpose hand saw perfectly satisfactory. But make sure it's sharp. And, look after it. If you saw through a nail with it you'll ruin the sharpness>

I like to mark a clearly seen line and aim to cut right down the outside edge of the line so I can just see it after sawing. That way your saw is less likely to veer off line because you can see it at all times.

When sawing take your time. Hold the saw at quite a low angle(this helps to keep at straight line). Use long slow motions and let the saw do the work.

For cutting multiple lengths from sheet timber I tend to get all of this done by my timber supplier. Most good suppliers offer this service now (some do it free of charge). They have huge saws which cut lovely straight square pieces from large sheet timber (like MDF)

When sawing lengths of timber to size with a hand saw sometimes it's difficult to get nice square edges or accurate angles. A mitre block can be a great help with both.

A mitre block is simply a guide for your saw. The saw blade slots into the saw guide and allows very accurate cutting. In the picture right I am cutting 90 degree angles.(ie straight across). You can also see slots for different angles. The one pictured also has slots for 45 degree cuts(mitres) which have many uses, (e.g. if you are cutting beading to finish off edges of wooden flooring around a room.)

To cut curved shapes from sheet timber use a jigsaw.

The narrow blade allows you to cut tight curves. Hold the wood down firmly with your free hand as the jigsaw will vibrate up and down a little.