If a child is experiencing difficulties with Maths, there are lots of ways you can support them.
First, and perhaps most importantly, don't put them under pressure to perform. Telling a child that they 'must do better' at a subject which they're already finding difficult is very unlikely to improve things and could well make them worse.
Let them know that you want them to do well because you believe the subject is important.
Let them know that you're going to help them in any way you can.
Talk to them about their Maths lessons, and the sort of work they're doing. Depending on your own Maths knowledge it's quite possible you can help them gain a better understanding of anything they're not sure about.
Play Maths games with them. Getting them to work out the change expected from shopping, or on a car journey, adding up the numbers on vehicle licence plates, are two obvious but very simple situations where you could promote Maths.
Some children actually enjoy being 'tested' on their multiplication tables.
Buy them a set of basic Maths equipment - pencil, ruler, rubber, pair of compasses, some coloured pencils and a simple calculator.
Finally, if you found Maths difficult at school - or even disliked the subject - there's no need to share that information with your child. It's very easy for children to 'inherit' a dislike of Maths before they've had a chance to fully experience it for themselves. (A surprising number of young people arrive at school knowing full well that they're going to hate Algebra before they've had even one lesson in the subject.)