There are several points to consider when choosing the right signal booster for your needs. First thing you need to choose is which network you would like your booster to support. In the UK the Vodafone and O2 networks are on the 900mhz band, and this is one of the most popular, most sites also offer dual band or tri band boosters, meaning you can support more than one mobile phone network, however as you would imagine costs are higher.
Once you have decided on the right booster for your network you will need to consider what kind of area you need covered. Most signal boosters start at 100sqm and go up to either 1000 or 2000sqm. For most houses 100sqm will be fine, but for offices and warehouses you may need something larger.
Finally you should make sure booster includes enough wire to go from the external antenna, to the booster and then from the booster to the internal antenna. You will find most boosters come with 10m of cable to allow you to easily get your kit setup.