You definitely don't need to spend much money to begin getting fit. Walking is the easiest way to add some cardio to your daily regime, and for that you simply need a pair of sturdy trainers or comfortable shoes. You should aim to walk 10,000 steps a day to maintain a reasonable level of fitness; you can buy a pedometer for under £10 and wear this to track your steps. If you find your daily steps amount to less than 5,000 on average, aim to add another 500 steps each day until you reach 10,000.
Just 30 minutes of exercise a day is all that's needed to start, and you can break this up into 10 or 15 minute chunks if time is really tight. Taking the stairs at work or getting off the bus a couple of stops early will all add up. Also, if you choose an activity that you like (such as cycling or dancing round the house to your favourite music, etc.), time won't seem like such an issue, as you'll be enjoying yourself!