Bad grades as a freshman in high school can have a variety of negative effects, both on your present and on your future. While you can debate the relevance of grades for hours, the reality of the world is that people will judge your value based on what you do, including in school. If your grades are not very good as a freshman, you run the risk of getting off on the wrong foot, which can be hard to recover from and can have a myriad of negative consequences.
Bad grades in your freshman year can be indicative of a trend. If you want to go to college, but you do not get good grades in your freshman year, don't write higher education off. Indeed, colleges are more interested in how well you will do in the future, not how well you did in the past. So, if you improve yours grades year to year throughout high school, a college will likely see that as indicating an ability to learn and improve yourself. Admissions committees are unlikely to weigh your performance as a 14-year-old as more indicative of your future performance than what you do as an 18-year-old. Still, getting bad grades as a freshman in high school can be a difficult habit to break, potentially making the rest of your high school career more difficult in many ways.
Bad grades do not occur in a vacuum. Rather, they are a product of the classes you take. So, if you take a full course load of higher-level honors classes and fail one of them because of the overwhelming content, the consequences of this will not be as severe as if you take a load of standard-level classes and fail one of them. It is better to get a mediocre or poor grade in a hard subject than it is to get an excellent grade in an easy subject; people in the future will look at what classes you took in addition to what grades you got in them.
Your grades are not your entire personality. There's much more to you than a letter or number assigned to you in accordance with a one-size-fits-all marking scheme. If, for example, you have poor grades but excellent SAT scores and a plethora of leadership experience in extracurricular activities, you will still be an attractive candidate for college or whatever else you decide to do. This is not to minimize the importance of grades, but it is important to realize that admissions and employment committees are going to look at more than your GPA.
College is not for everyone. It may not be for you, and it's not the end of the world if it is not. If you get poor grades in freshman composition and English literature, but frequently set the bar at record levels in auto shop or art classes, you may want to consider a specialty school rather than a traditional four-year college for your higher education. If so, you can explore additional high school elective courses to help prepare you for your chosen career.
There are a number of other consequences of bad grades that may directly affect you. Your high school may not allow you to play on its sports teams, for example, unless you maintain a particular GPA. Some parents will also use punishment to motivate you. If you are playing a lot of video games, but get bad grades, it is not unreasonable for your parents to assume you are spending too much time playing video games and take them away. Teachers talk to one another too. Bad grades as a freshman in high school may dispose future teachers to think of you as a low achiever. This can be a tough label to shake, and can handicap you throughout high school.