Well, the content is very different. Although they are sciences the things you will study on physics have little to do (directly) with biology for example. If you are going to study the three of them, I don't know it really depends on you. The important thing is to start working. For physics you need a solid understanding of algebra and know something about trigonometry as it says on the course. Biology I don't think needs any prerequisites and chemistry I guess it doesn't either (at least the very early chemistry).

I'm biased here because I love math and physics specifically and I haven't done both chemistry and biology in a very long while (although I want to study them also). 

I would suggest you at least spend 1 hour on each subject (math, biology, physics and chemistry) in that order every day. But remember, this is at least. If you want to get good grades you know you will have to work hard. If you have any suggestions and don't like the suggestion I've done, tell me so.

Finally, please do not repeat the posts.

Hi! 

I think you should test your level in each subject first. For example, you mentioned that your math level is not good enough, so you should see of how much you don't know about it. I would recommend you the book "Basic math and Pre-algebra for dummies", since you didn't keeped a contact with school for so long. The language is easy and is full of examples. 

For Physics, biology and chemistry i would recommend you to study them interchangeably, because at some point they connect together. Also try to apply some things you learn in real life. For example: How a fan works? 

You can answer this by learning mechanics!  

In a nutshell, see of how much do you know about each one, try to see books that have an easy language, do questions, and most important remember why you started all of this. Don't give up! I believe in you :>