First of all, I would like to inform you that you are in for a wild ride. It is absolutely imperative that you remain calm. The last thing you want to do right now is induce a 'bad trip'. A good friend of mine in high school accidentally took LSD when he mistakenly dropped it in his ice cube tray whilst putting it in the freezer to 'save it for a rainy day'. However, this friend had taken it before and knew what had happened to him. So when the walls started breathing he didn't freak out too bad. It would probably be a good idea to call someone you know and trust to come hang out with you for the next 8-12 hours. If you have never experienced a hallucination, it is not a wise decision to remain alone.
You need to be absolutely certain that you accidentally took LSD and not something else. I am assuming that you have already positively identified the drug. Based on that assumption, I do not recommend going to the hospital. LSD absorbs into your system very rapidly and therefore eliminates most possible countermeasures that can be taken with other types of drugs, such as charcoal flushing and\or stomach pumping-either of which are extremely unpleasant. Drinking milk has been rumored to lessen some of the effects, but like any other rumor it is hit or miss. Avoid eating cheeses or drinking orange juice, as both of these have ingredients which can intensify the hallucinations.
Any feelings you experience will be greatly amplified, so you need to focus on your state of mind. These types of hallucinogenic substances are very potent, and can bring about blissful and ecstatic sensations as well as feelings of mortification and dread. You will feel yourself begin to drift into a euphoric state, and things may begin to seem a little odd. Focus your thoughts on good things like family, friends, good times, etc. You are in it for the long haul now, and it's best if you just try to go with the flow and make the best of it. Your perception of time will almost certainly be warped like the rest of the world around you, and a single minute might feel like an eternity. But rest assured that this will pass. The world has not changed, merely the way you are experiencing it. Sight, sound, smell, taste, and even the sense of touch will seem very alien. Some senses will even mesh together allowing you to 'taste' colors and 'see' sounds.
Inevitably you will reach the apex of your trip (called the peak) where the drug is at it's highest level of saturation in your mind. This is the most intense moment that you will experience, and depending on how much LSD you took, this moment could last for minutes or hours. After your peak, you will begin to 'come down', and things will slowly begin to return to normal. While there are virtually no long term physical side effects of the drug, short term effects include sensitivity to light and sound, a gassy feeling in your stomach, sore jaw muscles, and difficulty sleeping. Note that while these physical effects are very short term (1-2 days), the psychological effects of your trip will last a lifetime. And like anything in else in life, how you perceive your experience will be your reality.