We ask that people do not double post - that is, to have two consecutive posts in the same thread without a gap of 72 hours. If you think of something that you want to add, then click the Edit button which opens up your post so that you can type in more content. Sometimes people do 2 posts because they want to reply to two people. You can reply to as many people as you like in one post and to make it clear who you are answering, use the @membername convention as I have done. As you type the person's username a dropdown list appears and you can select the one you want (it's a great way of checking that you've spelt the name correctly) and it's then tagged. If the person has turned on that option in their preferences, they will now get a notification that someone has replied to them.
So on to your question. I believe that you should not even think of doing an openworld game until you have had the practice of making a more tightly focused game. There is a skill in creating the whole arc of a game and another one in finishing a game. To have something as potentially amorphous as an openworld game for your first project is unhelpful for developing those skills. You can have all sorts of game-making skills, but without those two nothing comes to fruition. It is a major reason why there are so many unfinished projects hanging about.
Next, when deciding what project, ask yourself (and be really honest here) what are your strengths? For your first game, play to those - if you find writing a coherent story with fully-fleshed characters, interesting and dynamic dialogue etc. really difficult, then don't go for a story-heavy game. It may seem obvious (in fact, it is obvious) but people do often ignore this. Your first game is not the place to try and become uber-proficient in every aspect. Certainly work to improve aspects of game making, but don't base the game on a weak area, whatever it is. For this example, perhaps do a dungeon crawler with minimal story instead.
If you check the first 3 or 4 pages of threads in this section of the forum, I think you will find a number of discussions which might help different aspects of making your first game. Even more if you have the patience to go through more pages! They will give you a wide range of opinions and a lot of advice.