Write more and more things and this will improve your writing skillls!
Here are 5 ways you can use to improve your writing skills:
1. Try rewriting a number of newspaper and magazine articles that you read: In the process, challenge yourself to write a stronger and more to-the-point headline and content and then ask someone to compare the two pieces of writing and let you know what they think of it.
2. Try reading stuff aloud: Especially the stuff that you pen down yourself. Hearing the words written by you yourself can go a long way in giving you a clearer perspective of how good your writing is.
3. Try finding the best place/time to write: It could be your desk, your easy chair on the balcony or on the couch by the window; it could be early morning when everyone is asleep, evening just before dinner or late at night. It is up to you to find out what place and time get your creative juices flowing.
4. Try thinking outside the box: The key here lies in being creative. For instance, if you're writing about a dad scolding his son, then write about it from two different points of view - one from the dad's side and another from the son's side.
5. Try writing every single day: The best advice of all. Don't let even a single day go without writing. Write something, however short or long, every single day.
Although this has already been answered, I would like to offer my advice on how to improve your writing skills.
For myself, I find that improvement comes from critiquing yourself as well as sending your work to others for feedback. When you write a new piece, ask yourself if anything is lacking - what could be added, what's unnecessary, is the grammar correct, etc.
Another invaluable way to strengthen your writing is to read what you've written out loud. It sounds tedious and pointless, but actually hearing yourself say the sentences and listen to where you pause for breath helps you to understand how well your writing flows. I learnt this whilst working on a sports project and found it greatly improved my work.
Lastly, the best advice I can offer is to look at others work - by reading up on others and seeing how they write for various topics will undoubtedly help you set the right tone and style for what you're writing about and for. You don't want to write informally when you have a formal, academic piece of content to produce.
Hope this helps!
Only one way: Write, write, write.
Don't be afraid to expose yourself to criticism. To be sure, mean-spirited criticism can be hurtful, but constructive criticism is one of the best tools any artist has at his or her disposal to help them gauge their progress. So, if at possible, try to write in an 'open' environment where others can give you their opinion, feedback or criticism. A creative writing forum or a public reviewing community may be good places in which to flex your writing ability and perhaps take it to the next level.
However, while at first it may be good to write anything and everything, over time you should try to find your niche. As you improve your writing capabilities, you will develop a style all your own; this style, in turn, will most likely be better suited to certain types of writing than to others. I, for instance, am more of a blogger or reviewer than a technical writer; another writer may be the opposite of me, and excel at technical writing while lacking the skill to tackle reviews or fiction. There is no 'right or wrong' way of writing - there is simply 'your' way. Remember that.
I noticed you mention your GCSEs in one of your comments. At that level, I would recommend that you prioritise opinion pieces and summaries, with perhaps a little creative writing on the side. You need to be able to string your ideas together in a coherent manner, and that does not always come naturally; however, the more content you write, the easier and more seamlessly your writing will flow.
At the end of the day, it is important to remember that writing is a unique skill, and that no two individuals will write exactly alike. You should also bear in mind that the only way to get better at it is to do it. A lot. So get cracking, and good luck!
Just keep writing. Only if you want.