Summarize the following:
Inspect the post attached to your seat's saddle. Locate the mark that indicates how far this should be inserted into the seat tube to achieve the maximum safe height for your seat. If desired, you can place it deeper than this for a lower seat, but always insert it at least this much to make sure your seat is securely in place. A seatpost that is not inserted deeply enough into the seat tube can easily damage or destroy your frame if you attempt to ride the bike. If you want a higher seat than the insertion mark allows, you will need to purchase this separately. First, grease the seat post if you haven't already. Slip it into the corresponding tube on the bike's body. Push it in until the minimum insertion mark is inside the tube. If grease isn't included with your new bike, any waterproof, hi-temp grease will work. You can purchase this grease at any hardware store. Depending on the make and model, there should be either a bolt or a quick release mechanism on the outside of the seat tube. Whichever one it is, tighten this once you've inserted the post to your desired depth. Secure the seat post in place so it isn't jarred out of place as you ride. Once the post is secure, move up to the seat's saddle. If needed, adjust its position so it's centered over the post. Then tighten its bolts to fix it.

Summary:
Locate the minimum insertion mark. Slip the post into the tube. Tighten and adjust. Center the saddle.