To make your images ready for competition you need image editing software. The most popular programme for photo editing is Adobe Photoshop. Photoshop is available as a full programme (Photoshop CS) or as a stripped down version that will suite most camera club members needs (Photoshop Elements). However, these programmes can get quite expensive. Fortunately there are some other free programmes which can do a similar job such as Picasa from Google, IrfanView and GIMP.
Picasa has a very simple interface and enables you to fix simple problems with your photos, crop your images, and organise your pictures. However it doesn't re-size images so you'll need another piece of software to do that. IrfanView and GIMP are powerful programmes which allow you to edit your images, crop your photos and re-size your images, something which is essential when preparing images for competitions.
One of the downsides of using a free programme is that although the features may be similar, they will work in completely different ways. If you're a novice then Photoshop Elements is probably the most useful piece of software because of the amount of advice available on the programme, however, if you're on a budget and don't mind learning a new bit of software it's worth trying Picasa to edit and IrfanView to re-size your images. If you then want more advanced features give GIMP a go.