First of all, if all your drains and toilets are backing up, then the problem will almost certainly require calling a plumber, as the problem is more serious than just one slow drain. If it is just the one shower drain, then the easiest thing to try first is to use a plunger to dislodge the blockage. You can also use a commercial drain clearer to dissolve the clog, although these chemicals can also damage your pipes if used regularly. A more natural alternative is to try baking soda and vinegar to clear the blockage. If those methods don't work, then remove the drain cover and see if you can visually identify the clog. Maybe you can fish it out with your (gloved) hand or with a hook (you can make one from an old coat hanger). If you are still struggling then the next stage is using a plumber's snake. If that doesn't work, then you will have to call a plumber. Most clogs in shower drains are caused by a build up of hair and soap, so will only get worse if not dealt with.