I´m aware for some people it takes years to get their first Map ranked, but where should someone actually start on their way to rank a first map? what kind of songs should you try to map at the beginning, which length, where to learn about correct and incorrect slider velocitys, what looks good what doesn´t, which hitsounds fit, which one do not?

I´m also wondering which difficulty to start on mapping, which is the easiest in order to learn to adapt them onto harder maps?
Best mapping skin?
I have mapped some years ago just 4 fun but I deleted all my graveyarded beatmaps and want to try again as a serious mapper with serious goals aka. get a map ranked one day.

So many questions, yet all the answers Im looking for are shattered on the internet and on discord servers / subreddits. Pls help

It's usually better to try mapping the types of maps you play the most (but i'd strongly recommend against starting with tech/wub maps). Most people just recommend to map generic anime openings since those tend to be in abundance on osu. TV size is a good song length to start, not too long that you might give up but also long enoough that you might build some consistency when mapping. Most people map with default skin since that's what ranking criteria considers.

As for the rest, there isn't really a objectively correct answer. Just map and ask people to give feedback on your map. You could just ask any active experienced mapper for help, so long as their about me page doesn't say that they are not accepting feedback requests.

You can map any song you want, but it's usually recommended to avoid anything with complex timing or complex rhythms, and avoid super high BPM songs as it will only make things harder in the beginning. The advice to start with shorter or medium length songs (like 1-2 minutes) is also good, but keep in mind that you don't necessarily have to map the entire song, so even if it's 3-4 minutes you can either cut it or only map half of it. This allows you to spend less time on a single map so you can make a new one instead, which usually helps you improve more. Regarding difficulty, it's probably easiest to start somewhere in the middle, like Hard or Insane, but anything works. In order to know how these should be mapped (what is "correct" and what isn't, as you put it), you should look at various ranked maps of that difficulty level and find ones you like. Then you try to analyze the map and replicate some of its aspects in your own map, which obviously isn't easy at first but you have to start somewhere. Especially for low difficulties, it's important to read and follow the ranking criteria, but there are also general rules/guidelines to keep in mind, regardless of difficulty. As already mentioned, getting feedback on your maps is important to become better at mapping. You can do this in modding queues (either here on the forum or on osumod), certain discord servers or even by asking mappers directly via DM. There is also a mentorship program you could sign up for, it runs 4 times a year but it's not guaranteed you will be picked. Lastly, hitsounding is something you usually learn a little later when you have more solid mapping knowledge, but there are certain guides that can help you if you're interested, such as the ones listed in this post or this more recent guide.