What does cannabis do you?

Your question is a little vague, but I'll risk making the assumption you mean short term effects. Rather than long term heavy use. Cannabis has many many different effects, depending on the person taking it, the environment the person is in, the mood the person is in, the amount of tolerance the person has developed and whether the person has been taking anything else amongst many other variables. Also the type/strain of cannabis can make a lot of difference in many cases. I have been using cannabis on a daily basis for 4 years now, in my own experience, people with very little tolerance to the drug and who are introduced to it in a very relaxed and comfortable manner (perhaps by a loved one as a couple at home, and not in a group) can expect to see everything as somehow more beautiful and defined. Minor OEV's (Open Eye Visuals) can often be expected, sometimes to the point of being able to make anything you look at take any shape you fancy. (Cloud watching can be an amazing experience to a 1st time user) Also a feeling of tiredness, and relaxation, also minor confusion. A greater appreciation of music (even music you normally would hate) can usually be felt. Expect to fall asleep if overly comfortable. However in a different environment, expect this to change completely. In an environment around a group of people you thought you were comfortable with, you could feel extreme paranoia, an inability (or lack of want) to move or even talk. Sitting in a silent state of fear that you don't quite understand and can't make sense of due to not experiencing it before, and the confusion that coincides with fight or flight responses only makes things worse. Cannabis can also make logic VERY difficult (Maths is a classic example.) However it can stem all the way down to basic thought processes, further amplifying the confusion/paranoia problem. In serious cases one can often expect to feel scared to even do something as simple as step up a curb or even stand up, for fear! however fear of what, you may not be able to figure out. As tolerance develops one can expect less of the paranoia however this can take some time for some people (even years). However even after 4 years of daily use if I find myself in a situation high, that I'm not used to being in high, minor paranoia can still easily strike. Other very common side effects are:
Munchies:
You find you are constantly hungry no matter how much you eat, you can eat to the point where you feel you should explode,... and then happily carry on eating. This tends to slowly subside as tolerence develops. Fear of the Dark:
Even someone with a high tolerence, when faced with walking down stairs in the dark can find themselves being so overly cautious it can take minutes to actually get to the bottom! Giggles:
Generally comes more with Sativa dominant varieties more so indica, however users with a low-medium tolerence tend to get this the most even from indicas. Often at parties where spirits are already high, or in situations where nerves but not fear can kick in. One can find themselves in public places with completely uncontrollable fits of laughter that can last for what seems like an eternity. Heavy users don't tent to get this as much, especially with indica varieties. Short term memory loss:
You will often find yourself completely forgetting what you were about to say, right in the middle of a sentence. As soon as you realised you've forgotten what you were about to say, you may also realise you've forgotted the topic of the conversation you've been having for the last 45 minutes. This can be very frustrating, but equally hilarious. This tends to be an effect that takes a high tolerence before it begins to really fade. Bloodshot eyes:
Most users find they get redeye. Eye's feel dry, and red, Not sore but sometimes a little itchy. Following Redeye is slit eye. Where the eyelids also begin to close. and you find yourself being able to see perfectly, even though your eye's look almost completely closed in the mirror. This also subsides but usually only with very high tollerence does it go away.
I do not want encourage cannabis use, but I won't slander it either. I can only speak from my own experience, so please bare that in mind. I hope this has been of help to you and others. Note: Mixing cannabis with alcohol creates something entirely different. (Even if you have a very high tollerence to both seperately). You can almost garentee an unpleasent experience until a 3rd tollerance is developed. A tollerence of them both combined. This has caught many people off guard. Be warned.