I'm a real noob and Dota2 is my first MOBA so I suck at all levels, though I have a couple friends who tutored me past the most glaring beginner mistakes, thankfully, so now I can scream at other noobies for fucking up my lane.
Anyways, I've taken a different approach to learning. I've tried to play most (aka 50% +1) heroes at least once so I have passing knowledge of their skillset, something you can't really infer from playing against them. Their speed, skill mechanics, usual combinations - it's important to understand how your opponents are likely to act because you can then anticipate them, which can basically add up to a kill for you or them. As mentioned earlier, the importance of an early game gank can't be overestimated and therefore it's critical that you can also avoid one.
Which leads me to the softer Dota skills. Warding, last-hitting, denying, stacking, jungling... you can't learn these things in a bot match. If you don't want to fuck up a public game, there's a lot of video material to learn from. Typing these terms on youtube will net you a lot of tutorial videos, of which I can recommend Purge and XVRogue's. Having a basic theoretical understanding of any of these is essential since, again, it's all too difficult to pick them up during a game (when you're also 110% concerned with other things, such as not dying). The exceptions to these are last-hitting and denying since bots, especially on Hard and above, are pretty exact on their calculations and so practicing against them will actually make you a better finisher. But they are still limited since they generally don't harass you and you can spot their ganks from a mile away.
The best beginner skill to learn, in the end, is how not to feed. And this one requires actual humans to learn. People are more unpredictable than bots, for better or worse. Sometimes this will mean that the enemy team fucks up and wastes an opportunity to stunlock and kill you, while other times it means that your lane partner just walked off to greet a courier, your mid didn't call mia, and you're now in the middle of a painful gangbang.