This is simply a guide to build your own adventures.
In order to create an effective role play, you need to first create a believable setting. Without such, you are doomed to rejection and failure. If the player's don't believe the world they are playing in, they are liable to quit, abandoning the RP as though it never existed at all.
To begin with creating the setting, I suggest you open up MS Word or get a pad and pen and write down everything you want in the RP, then play with them a bit until you create a decent amount of information. Then play into that by justifying everything you wrote down in some way. Let's say the first thing I wrote down was "People can cast spells." I need to answer the questions of Why and How, and it might also be a good idea to answer When (or Since When, in this case).
So, my new sentence is, "People have been able to cast magical spells since the middle ages because of the discovery of Magic Stones that give humans such powers." The question of How is vaguely answered, but this is good enough for now.
After you complete this task, you should think of a story or driving force. If you choose a story, sometimes called a Campaign, you must careful decide where the players must go and who they must speak to or kill or save. If you choose to simply have a Driving Force, then you must want a more dynamic story that will play out through the players' actions.
Stories need to have starting and ending points, which include locations, times, and events. Force driven RPs only require a Start, but may have an End point predefined as well. The only defference between the two game types is that one path is entirely up to the Game Master, while the other is up to the players.
I will only cover Stories this time. It is best to begin with a story premise to show what the goal of the campaign is. Whether it be "Save the Princess from a Dragon" or "Kill the wicked King of Lalalalala", the premise just has to be short and to the point.
Next, you must map out the adventure, preferably in a linear format of locations. These locations must be visited by the players for them to progress through the RP. You may want to add a few extra forests or towns the players can choose not to do, to vary it a little. If you want the story to be a little more dynamic, and fun for yourself, then I suggest also creating alternate paths for the players to follow. This way you will get to be surprised to some extent.
Throughout the story there should be conflicts. In order to keep the attention of the player's, you should have random attacks or conflicts when the characters aren't in the middle of deep conversation. Then when the players are talking without any worry of conflict coming from outside their conversation, you may once in a while create a conflict to throw them off. Don't be predictable, or static. Be dynamic in everything you do. That way, as the RP progresses, things change. A player may die or become ill (or quit), and the others will have to face that. Use NPCs to your advantage as well. A NPC can be used to force talks between important characters or about important subjects, and can be used to progress or hinder the story.
Don't be a push over. If the player's start to do things you have already explained to be against your rules, then punish the players. This may affect your RP dramatically, but can be used to add to the dynamics of the linear story.
About now you should be planning the end of your RP. Make sure the last conflict, the climactic battle between the players and their antagonist (an other player or NPC), is drawn out. Don't let one side or the other win too quickly or easily. If one side is winning to soon, give the other side a helping hand, and vise versa. Keep it going until you and the players want the conclusion. Then let them finish it. The final post can be a player's, or your own. If it is a player's, let it end with a a speech or a even a spoken sentence. Otherwise, end it with a conclusion that explains what happens to the players' characters and how they affected the world or the setting, if at all.
I hope this was helpful. It was my first, but I plan to get better. Peace!