If you are looking for part time data entry jobs then your first step should be to contact your local job agencies and get yourself registered with them. If any data entry jobs come up they will put your CV forward to the potential employer.
You may however be looking for data entry that you can do from home, maybe in the evening or a few hours during the week either morning or afternoons. Maybe you are looking for work that you can do to fit in with school hours. You can check the online equivalent of your local job agency for jobs that are open to home workers. Many employers do outsource there data entry to home workers. Many UK employers prefer to outsource to typists in the UK as they feel they have better control over their data and they have to comply with the UK data protection laws.
You should of course be wary when applying for data entry work online even when you apply through legitimate online agencies. Many agencies will list third party job ads that they do not vet and so may not be legitimate.
When looking at job ads always do your due diligence to make sure that the jobs are legitimate. Many part time data entry job adverts are no more that vaguely disguised methods to get you to part with your money. They may ask for a fee to register you or for a membership or even charge a fee for the promise of some work. It is recommended that you do not send the any money, you should not have to pay for any kind of job.
If you are looking for typing based work that you can do on a part time basis from home then one option is to do paid survey work. Many opinion companies will pay for you to complete surveys on different subjects, in return they pay you a fee.
You do not have to pay anything to register your interest and you can often start earning straight away. The amount of work and the hours that you do are completely flexible and it is a good way to start earning some pin money straight away.