Well, it's pretty simple. Up near the top of the page there is a link that says 'Serebii.Net Forums' you can click that. The forums page will have many section and sub sections like 'Pokemon Anime Discussion,' 'General Pokemon Discussion,' and so on. Click the title of one and then you will have another page will possible sub-topics at the top and threads below. All you do is click one of the threads and there should be a responce box at the bottom of the page, type whatever you want about the subject and then hit reply. You can also pick 'Go Advanced' to have more options on your post like disabling smilies, font color and other things.
I think I made it sound more complicated than it is, but in order to post, just look for that responce box.