@PikaPikaGamer ...
It rather depends on the content. If it's pictures, I would recommend one of the many hosting sites (I use Imgur, but there are others, such as Photobucket, Flicka, ImageShack...). Upload the picture to there, then use the BBCode link that they provide. This simply gets pasted into your post and 'Hey Presto..!', there's the pîc..!
It's a Good Idea, especially with larger pics, such as screen shots, to use 'spoiler' tags around the pics; it makes reading through threads so much better for all. Ask if that's a mystery to you...
To host BGM files, the best solution is to create an account with Soundcloud (it's free for light usage...), upload the fil to there, then post the link provided in your thread. Soundcloud files will embed into the post, making it easy for anyone to try it out without having to download it first.
Is that what you wanted to know..?