Evaluate the problem. Getting rid of a little mildew on the bathroom tile is no big deal. Dealing with an entire room that has been taken over by it is another matter entirely. If the mold problem is extreme, then don't waste your time. Call a professional, as you could be putting your health at risk.
Eliminate the moisture source first. Wherever the is mildew, there is moisture. Eliminate the moisture, and the mildew will no longer be able to grow. If the problem is in a bathroom, consider improving the room's ventilation. If the problem is in another room, check for water leaks in adjacent rooms and in the attic or the room above. If there are no leaks, and you live in a humid environment, then consider running your air conditioner more or putting in a dehumidifier.
Purchase a chlorine bleach-based cleaning fluid intended for getting rid of mildew. Your local grocery store or discount store should have a variety or products available in different strengths.
If the mildew is on tile, porcelain or grout, then simply apply the cleaner, let it stand a few minutes, and wipe it away.
If the mildew is on wallpaper, be aware that the bleach may remove the color. If the problem is bad, it may be necessary to replace the wallpaper entirely. Be sure to wear goggles and a face mask (rated N-95) for this job.
Mildew that has grown into drywall can be particularly insidious. If it is just a little, then cleaning fluids may be enough. If it is deep-set, then it may be necessary to replace it. Consult a professional. Do not paint or wallpaper over damaged areas unless you get rid of the mold first and eliminate the moisture source. Otherwise, the mold will still be there contaminating the air you breathe.