There are a lot of factors in placement.
I believe you either You start with your normals hidden MMR, or maybe something like 1200 MMR, and have it adjusted game after game. Besides the basic of the MMR system with winning / losing streaks, there are just a lot of other factors such as which side you are on. or example, on blue side, your team has lower average MMR, and you will win less per win / lose more per loss due to the blue side camera advantage. But yeah, streaks are a huge factor since there really isn't that large of a MMR gap between the different divisions.
The placements based on win/loss posted by ZYRRA looks fairly accurate. And as mentioned, your actual MMR may be quite a bit higher / lower than where you are placed where you will be having high LP gains / low LP losses or the other way around if you are placed higher than your MMR.
In terms of duoing with people, you will be placed against opponents of a bit higher MMR to compensate.
For reference, here is where I placed
S3: Bronze I, 4-6. Very high LP gain (like 30's) but I was never able to win promo into S5, so it was probably where I belonged at the time.
S4: Silver I, 7-2. Started 1 - 1, lost a game somewhere in middle, got a loss prevented due to server issues somewhere else.
s5: Silver II, 4-6: Did pretty badly, started 2-0, but went on 3 game losing streak. Same thing for last 5 games.
Honestly, don't worry about it too much. Just keep on playing, have fun, and get better and climb naturally.