Tracking 10,000 Steps a Day
Probably the most simple and important thing Fitness Trackers measure are "Steps". The good news is that you don't need to spend a fortune on a device that can measure the steps you take throughout the day. Simple Pedometers as they are known can be purchased for less than £10. Just using a pedometer to count your daily steps and aiming for the accepted 10,000 steps a day target (though where this target comes from seems a bit sketchy! see this article) could be enough for you to get on the fitness trail. However, you may find that using a pedometer is not too convenient as you may need to manually note down your daily totals which can be a chore and if things are a chore, you may give up and be back at square one.
If you have a smartphone, there are Apps that can act as Pedometers and since you may have your phone in your pocket a lot of the time this could be a good solution for you. These are likely to keep a daily record of your Step Count and make this aspect easier for you. You will need to think about whether you will actually have your phone on your body all day AND you'll need to remember to start the App too or you will lose data.
The more sophisticated (and expensive) Fitness Trackers carry out the same function as a pedometer, some that are wrist based are designed to be worn all the time and therefore do not need to be started or stopped like an App and will constantly gather your step data. Some devices will be able to indicate to you your step count directly and many of these will securely send the data to the internet where you can analyse it on a web page or smartphone App. A large part of the benefit of a dedicated all day worn Fitness Tracker is that it quietly gathers data all day and night (unless you take it off when you go to bed) and then provides you with a rich set of data from which you can see your activity levels. this is much less of a chore than any of the other methods.