Maths is all about practise. You need to be doing past exam questions, especially focusing on topics you're weaker on.
Since you've moved to higher maths, I recommend using the specification as a checklist for topics you've gone over so you know what you still need to focus on.
Maths Genie has plenty of past paper questions and they're really useful because they're separated by topic, and the topics are separated by grades (go to the mathsgenie website and go to gcse revsion, then you'll know what I mean).
For content you don't understand, I found youtube videos very useful. GCSE Maths Tutor and Mathsgenie as great videos on nearly every topic and they go through questions of different difficulty. GCSE Maths Tutor had a video called 'everything you need for higher GCSE maths', so this is a good video if you're concerned about the new content that comes with higher maths.
To make sure you're exposing yourself to as many questions as possible, I recommend Corbett Maths 5 a day. It's 5 maths questions a day and there's different 'sets' based on difficulty. You need to do as many questions as possible for maths as the types of questions tend to repeat, just with different numbers, so the more questions you do the better prepared you are.
If you have any other questions I'll be happy to answer the best I can