Bit vague mate. Firstly you need to know the subject and then know how to work with say PowerPoint.
You need to know what the course is for, who it is targeting (so you get the content pitched correctly), what are the desired outcomes (sounds a bit poncy, but basically what is the training expected to deliver as sometimes this can greatly influence what you put together or start an argument about what training can & can't do!), what are the measuring requirements (test / practical etc).

But more importanly you must be able to deliver a good, fun, interactive course for those who would be attending. I've been on the receiving end of some terrible training, where the trainer just reads the slides back to you, reads from pre-prepared notes (think a politico giving a speech!), has no interest in deviating from the delivery and so on.

If you aren't confident in delivering to an audience and knowledgeable on the subject in its entirety don't do it. If you have never delivered training before, more employers now (where training is a large part of the job) are looking to the PTLLS qual. It is an nvq3 - bit patchy but worth thinking about. Sets you apart from those who say they can deliver training and those who have got a qualification that includes an assessment of you actually delivering some kind of training (usually a 30-45 minute delivery on a subject of your choice but taking into account all you learn during the week).

Bottom line is don't do it if you think it'll be only to improve your job prospects. Do it beacause you are confident in delivery and enjoy training as if you aren't, it'll soon become apparent. Me I love it as it is all eyes on me & you get some good banter going even when I was teaching NEBOSH Cert etc.