Hi!
Starting off with a personal song choice is really ideal, but it must be really easy to map. Meaning to say, you're able to catch up on how and which to place hit objects through the song beat and its tempo, as well as how long does it take to finish the map. In general, choose a short song ranging around 2 minutes is a great start, with a suggested tempo of 180BPM.
Next, Feel free to map and make trials and error. It's your first time so it should be a learning process for you. Don't worry if it's sounds terrible as long as you know which one isn't that way and such. You may also like to get feedbacks from this
subforum - a place where mappers get feedbacks by joining in one of the opened threads. Each thread has its own rule so be sure to follow otherwise they reject your map. And not to mention to check out the main rule in that subforum
here.
Last but not least, take inspirations. Feel free to check out the mappers that you really like and found their mapping style suiting for you. You can then apply with your own style as well a bit from them to see how well it goes. I would suggest to have lots of maps as you progress so that you can get more ideas on how each mappers mapped their maps accordingly.
Overall, mapping takes time. You really need to be patience in creating a great map because it consumes time - not only in minutes or hours, but it can also be days or weeks. As long as you're determined to start a map, you'll be alright.
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osu!mapping tutorial by Pishifat