The first goal is to diagnose why the post is stuck, and the first thing to do is to completely remove the seat binder bolt. If your bike has an external clamp that is separate from the frame, it is worth removing the bolt and sliding the clamp off of the frame and loosely onto the post. Check to see if removing the bolt loosens the post, and for any signs that the clamp was overtightened and crimped the post in place. If you have had the bike for a while and haven’t crashed since your last seat adjustment it is safe to say that your seatpost is the correct size and unlikely to have a subtle bend causing it to stick. Used bikes are another story—it’s amazing how many people force an oversized post into a frame only to wonder why it won’t budge later. If you do suspect a mechanically stuck post it’s best to consult a local shop or framebuilder for further advice.
Some sort of oxidative or galvanic seizure is more common, and the most likely culprit for a metal seatpost in a metal frame that was just fine last time and is now stuck in place. Without getting into the metallurgy of each—an oxidative seizure is caused by steel or aluminum corroding and the oxide layer being thicker than the original material, galvanic seizure is caused by an electrochemical reaction between dissimilar metals causing one to corrode to the other. No matter the case, if a good twist of the seatpost from your strongest friend doesn’t make it budge your best next step is to turn the frame upside down and use a liberal amount of penetrating lubricant applied to the inside of the seat tube to hopefully break it free. There are a number of commercial products meant for freeing stuck bolts and the like, but in their absence any thin lubricant should help. Follow the manufacturers safety instructions, and then let it sit overnight before trying to twist the post free.
If the overnight bath doesn’t work, you suspect the post might be mechanically stuck, either your frame or post are carbon or you otherwise feel in over your head it’s a good time to start calling around to local shops for an ace mechanic’s help. Methods beyond a penetrating lubricant become more creative, and more destructive, fairly quickly and it’s best to do your homework and let someone with more experience actually do the dirty work.