Picking out a theme for your home is all about you - what you like, and what you'd like to live around. The actual selection is easy - it is adapting it to your home that is the difficult part! Common themes for homes include Asian/Japanese, retro and rustic styles.
Once you have selected a style or a theme, you will need to consider what furniture you will need, as well as what colours you may need to use. In most cases, furniture appropriate to the theme can be purchased either new or used, though for some themes or furniture types, you may need to manually change the look of the items - perhaps by painting, varnishing or sanding in the case of wood products.
The chosen style or theme may be used in a single room, such as a bedroom or living room; or it may be carried throughout the entire home. You may prefer to have a 50's 'retro' style in your living room and dining room, while keeping your kitchen area much more modern and practical. However, be careful that you do not try and add or combine too many themes in your home, as this can cause clutter, and thus ruin the effects of the theming.
Another caution when theming is not to make themes appealing to a wider audience. While a sci-fi or football theme may be brilliant for you, when you come to sell the home, potential buyers may find it a turnoff.
Of course, you may not want a specific theme such as an Oriental or Modern theme, and you may instead simply opt for a theme derived from certain colour combinations - the choice is yours. This is also much simpler, as you probably won't need to purchase any additional furniture.
At the end of the day, the style of your home can be as unique as you are - the only limitation is your imagination! Though it is important to remember that you will have to live in your newly styled room - if you are likely to get bored with a style or theme, it would probably be wise not to start it, as you will only incur the expense of changing it again later on!