So you want to improve your grades. Congratulations on taking this first important step toward academic success, and tons of cash further down the road!
The first thing you need to do is to read the syllabus for each class, and identify exactly where the grades come from. How important are homework assignments? Exams? Projects? Class participation? Know where you stand so you can focus your efforts on the areas that need improvement.
Generally, a class is designed so that if you learn the subject matter, you'll get a good grade. But how can you learn the material? That depends on the type of subject you're studying.
If you're studying a foreign language, the best thing you can do is to memorize vocabulary by taking your vocab list and writing it over and over again, until you can write the entire list from memory. You may have to write the list over five times, maybe ten- but I guarantee that once you can do that, you'll ace the exam.
If you're studying math, then the best thing you can do (aside from going to class and completing homework assignments) is to be on good terms with your teacher. As soon as something looks confusing, don't be afraid to go up to him or her and say so. You may need extra help after class, or through a private tutoring program, and that's okay! We all have our strengths and weaknesses. It may take a little extra effort, but you CAN learn it!
The same applies to Science classes. Do the work, and spend time memorizing the material by writing things over and over. Do the reading. Again, the amount of time you spend honestly trying to learn and become familiar with the subject matter, the more comfortable you'll be with it on test day, and the more your grades will improve!
As for History and English, it's all about the reading. Take time to sit down and really get into it. You'll make your life a whole lot easier if you just take the time to read (and sometimes reread) the assigned readings. Become familiar with them. If you have to memorize vocabulary or dates, again, write them down until you can do it by heart, and you will succeed!
Lastly, and perhaps most importantly, is motivation. Class can seem boring and pointless, but don't forget why you study and get good grades. You're setting yourself up for success later in life. Colleges and Universities look for people who are willing to devote themselves to their coursework and do what it takes to succeed!
By putting in the time and effort to read, memorize and practice the material, you're setting yourself up to get the job and the life of your dreams further on down the road.
Remember: time invested in your studies is time invested in yourself. Work hard and take pride in your achievements. You'll be glad you did, when you're rolling in piles of cash and lighting cigars with 100 dollar bills.