Read a lot. Be it novels, non-fiction, newspapers or magazines, you absorb knowledge of correct grammar structures just by coming across them in your reading. Listening to audio-books also has the same effect.
Focus on the elements of grammar you find most challenging. Ask an English teacher or tutor to explain to you the grammar structures you're having trouble understanding. Practice using these structures in your writing and everyday speech.
Do grammar exercises. Search for
"grammar exercises" with an Internet search engine to find ones you can do on-line. Ask schools for copies of old grammar exam papers or get a teacher or tutor to design exercises specifically for you.
Type words and phrases into an on-line concordancer to see examples of how they are grammatically used within short sentence extracts.
Read and work through a math textbook that includes exercises. If you've never learned math, start with the beginning or basic sections such as addition, subtraction, multiplication and division. If you have general math knowledge but want to increase your competence in particular aspects of math, read the relevant chapters of the the textbook. Work through the exercises the book gives you as part of each section.
Practice solving different types of math problem. Practice makes perfect but if you keep working out the same kind of problem, your knowledge of math will be narrow and you'll become bored with your work, leading to sloppiness and mistakes. Ask a teacher, tutor, local school or homework club for past exam papers, as real papers have a variety of questions. Alternatively, search on-line for math-dedicated websites as they'll have different sections with exercises for each branch of math.
As you get into the habit of solving more and more problems, remember to check your answers. You will not only find any mistakes you made but hopefully will be able to figure out where you went wrong by yourself, instead of waiting for a teacher to mark the paper and tell you where you went wrong.
If you find a mistake but don't understand where you went wrong, spend time clarifying the point until you do. Clarify the point by either looking up the appropriate section in your textbook or going through the exercise with a math teacher or tutor.
Once you're more confident in your math ability than when you started learning, try solving timed math problems. Either get worksheets from a teacher, tutor or school and time yourself or go on-line to find automatically timed exercises.