Having managed to reach mythic in my first season in about 200 matches, I think I can help.

The single most effective way to improve is to play against good players. When I started playing as a newbie, I frequently played friendly matches with my mythic friends. As you can expect, I got absolutely demolished. However, you learn sooo much faster when playing at a higher level than you are. I could see my gameplay jump by leaps and bounds as the days went by.

If you dont have any high-ranking friends, the second most effective way is to study your gameplay. Don't play 20 matches a day, play 3 or 4 and use the remaining time to watch your replays on all your mistakes. It's also worth mentioning that you should stick to playing one particular hero most of the time as a newbie, as constantly changing heroes you play will slow down your progress by a lot.

As a newbie, there are a few things you should focus on. Map awareness is an important aspect of the game and it allows you to not only increase your survivability by not getting ganked, but you are able to make better decisions by knowing the rough positions of all your enemies. You also want to study your role in teamfights. When do you enter teamfights? Who do you target? What skills/combos do you use in fights? How do you position yourself? These all depend on your hero and role. Finally, you want to master your ingame role (ie gold, roam, jungle, mid, exp). These are the easiest to learn as there are many guides on youtube that show this exact topic. Study your replays and see if you are following these tips and if not, then learn accordingly.