If you want to make extra money alongside your normal job, then there are things that you need to be aware of to make sure that you are acting legitimately.
To start with you will need to check with your current employer that it is okay for you to do extra work. Some employers are perfectly happy with this, but others may have specific rules in their employment contracts to make sure that you do not earn from anyone but them. Some will only restrict work that is related to what you do for them, so doing something completely different to earn money is fine, but others will state that no work can be done outside of your job as every idea you have, whether in work or outside or work belongs to them, even if it is completely unrelated to your job. It is always best therefore, to just contact the relevant department at your place of work and find out whether they have objections to you doing some extra work in your spare time.
The other thing that you have to be aware of is tax. All earned income is subject to tax and this includes any work that you do outside of your normal job. Therefore you will need to keep a record of what you have earned and declare it to the tax office. You can do this by registering with them and completing a self-assessment tax return each year. It is simple to do and you can do it online. You get asked a series of questions in a step by step manner and you need to just complete your employment income (which you will get on your P60) and your self-employment income which you should have noted down as well as any other income such as interest of savings accounts and investment returns.
As long as you do the things above then you will be able to earn extra money within the law. There are lots of ways that you can do this and it is worth noting that some are subject to tax and some are not. If you sell items that you previously owned, then you do not have to declare that, but if you buy things and sell them, then you do. If you win money through lotteries or earn cashback on spending you do not have to declare it, but if you get paid for answering surveys, watching videos or doing freelance work, then you have to declare it. You can take expenses away from self-employed earnings as long as they relate to what you are doing, so if you set up a website to sell things through you can take off the cost of the domain and hosting of the website. If things get too complicated with regards to earnings, you can get help with book-keeping but paying someone can be expensive, but as long as they save you money in tax, then they are worth it. So only get one if you are earning enough to be paying tax and paying a significant amount.