Welcome to the thread!
Great! Glad you want to start building. I recommend that you watch this video series that I made:
Then I recommend that you download LineageOS 14.1 (Nougat), and build for your device. You can find your device trees here:
That last link has your vendor and kernel files.
Notice that in the videos, the first 7 videos are specific to downloading, setting up, and building Nougat. Then video 9 will help you add your devices to the local manifest.
@aapav01 Built this device tree already, so it should build good for you. You can slap his thanks button in his thread here, for all of his hard work:
Then, once you have got a working Nougat build, you can work on Oreo, and perhaps Pie. I do have videos for upgrading a device from one version to another, as well, and it looks like aapav01 has started some lineageOS 15.1 builds as well.
Hope that gets you pointed in the right direction! Let me know if you have more questions.
Is it possible to start afresh without using any existing device tree?
I mean is it possible to compile TWRP recovery and Lineage OS ROM from Lineage OS'/TWRP's source code and Device's source code(includes vendor files from Samsung) ?
Also the second video of the playlist builds the ROM for "jfltexx" for which the tree already exists.
What if the tree doesn't exist, or if I want to build for a completely new device(port a new device to LineageOS)?