Microsoft PowerPoint is a program that is part of the Microsoft Office suite package. If you can manage to get a copy of Microsoft Office then you are guaranteed to have a copy of Microsoft PowerPoint. Microsoft Office is a very expensive program, though, so it will take some looking and ingenuity to acquire a copy of that or Microsoft PowerPoint for free.
If you are a student the first place to look if you are wanting to get a free download of Microsoft PowerPoint is your school. Many high schools, colleges, and universities have agreements with Microsoft that allow them to offer a free copy of all or part of the Microsoft Office suite to their students. Your instructors are unlikely to know about whether or not the school offers this sort of software for free. A call to your school's technical support is likely to yield far better results. Your school's technical support should not only be able to tell you if your school offers such software for free, but they should also be able to assist you in downloading your free copy of Microsoft PowerPoint to your computer.
If you are not a student or are unable to get a free copy of Microsoft PowerPoint from a school you should check with your employer. Many employers have similar agreements with Microsoft. Calling technical support is also generally the best route to go about finding out if this software is offered and getting assistance in installation. If this isn't an option, either, you may wish to get in touch with friends and family to see if they have a copy of Microsoft PowerPoint that they could let you install on your computer. Generally, if your friends or family purchased a copy of Microsoft PowerPoint they will be able to register it on a few computers. If they have only registered their copy on one or two computers you may very well be in luck.
If you only need Microsoft PowerPoint for a short period of time you may also want to check out Microsoft's website. They almost always offer a free trial that will allow you to download Microsoft Office and use it for a short while.
If these aren't options for you, you may want to turn to the Internet. Microsoft PowerPoint is available online from several websites to download for free as torrents, but the legality of this is questionable, so download at your own risk.