Study your syllabus. This has a list of all topics covered for that module. As you study tick off what you have covered on the list.
Do your own research. You’re in college now not school it is up to you to do most of the work. This means reading books or watching youtube videos on the subject to help build your knowledge. I find watching youtube videos helps me study a lot they are offend thought by other lecturers who may explain things more simply and clearly for you.
Joining a study group is a great way to get extra help. You can ask your peers about anything you don’t understand. Asking each other questions about the topic is a great way to study as you have to be able to firstly understand the question to ask it and secondly to answer it.
A lot of colleges offer extra classes to help students especially in first year. My college has extra chemistry classes for first year students called ‘peer assisted learning’ it is thought by second year students to first year students. It helps students with any areas they may struggle with especially if you have never done the subject before.
Talk to your lecturers or email them if you are shy. Explain that you are struggling. Lecturers are there to help students learn often they can direct you to a website or book which could help you catch up or even put you in touch with a second year that can help.
The most important thing you can do if you are failing is to read past exam papers. As I have said before lecturers keep to the same basic structure each year so if you can do the past exam papers you will pass the exams. It’s also a great way to find the gaps in your knowledge.