Curtains can say a lot about a room. You'll find they also say a lot about you. Some people I know change their curtains every year, some never. Whatever you do you should consider if they're right when you're trying to sell your home.
Blinds are an attractive choice for many people - and can be bought cheaply online. Pick neutral blinds that fit in with the current decor.
If you've got curtains then consider washing them (or dry cleaning) and ironing them - or just beating them whilst they're hung over the washing line to get the dust off them. A touch of freebreeze can liven them up to remove any stale smells, but if they're yellowed or dirty you should replace or get them cleaned.
Nets can look nice when they're brand new - they can be a cheap way of covering windows that have slightly offputting views.
Voiles are thin smooth fabrics used to dress windows. They can look very attractive.

If you're looking for cheap options check out charity shops for curtains - you might find some lovely ones that match, or look online at cheap curtain shops.

By dressing your windows you make your home look more attractive.