[John_Toolbox]

We need more information to help you.
What is the subject matter of the video you need music for? (Advertising, tv drama, etc.)
What type/genre of music are you looking for (orchestral underscore, electronic underscore, high energy, action cue, etc.)
Budget/time constraints?

There are really 3 ways to go about this:
1. Purchase a DAW like logic or pro tools. Purchase sample library software(the included content in pro tools and/or logic is good, but is not enough to cover everything you will probably need). All of this software has a high learning curve, and being able to use it doesn’t guarantee you will be able to create high quality music. This route will have a lot of time and money invested if you are not familiar with the process of creating music.
2. Look for a royalty free music library company and listen to their content until you find something to shoehorn into your project. This is the cheapest, fastest way... but will not have the best results.
3. Hire a composer that has good audio skills and owns/knows all of the software needed to create your score from start to finish. You will probably get an overwhelming amount of replies if you post an ad saying you are looking for a composer, but finding one who has all of the needed skills and has the “sound” you are looking for is more difficult. There are quite a few composers like myself that work remotely, so you have a lot of options. I usually ask potential clients for a short clip of the video they need music for and score it to show them what they would be getting.