Welcome! Glad to hear you’re enjoying yourself. 
A quick look at your armoury suggests that you’re starting with the right things. World Quests open, several Heroics done, Arathi Warfront done - don’t miss Darkshore Warfront when it’s next up - all that.
So you’ve been on the right track, and you’ve got very solid 355 gear.
So bookmark this. You’ll be coming back to it:
You understand how M+ works? A very short summary:
- You do a plain Mythic, aka M0.
- From that, you get a Keystone, usually for a random different dungeon +2.
- You get a pary together, travel to that dungeon, enter on Mythic, place your Keystone into the font (there will be one just inside the entrance) and then your party tries to clear the dungeon as fast as you can.
- If you succeed, you get 1, 2, or 3 chests, depending how fast you went, and a new Keystone for another random different dungeon at a higher level/
- If you fail, your keystone gets reduced one level.
Now, you can also join someone else’s key. That is, if someone has a Keystone, you can join their group and do the dungeon with them. In that case, you will get a chest, but your Keystone, if you have one, will not be changed at all. If you didn’t have a Keystone before, you will get a Keystone form that run.
At the end of the week (which is early Wednesday morning, btw!) you will find a chest in the Harbourmaster’s office, with a piece of gear reflecting the highest M+ you did last week.
355 is OK to start M0 and low M+, but your experience is sketchy. Since a failure will downgrade the leader’s key, it is only polite to be sure you have at least a basic grasp of each dungeon on a lower difficulty before trying it in M+. People get really tetchy if you start going the wrong way, or pull an unnecessary pack of mobs.
Here are some groups where you might find friendly people to accompany you.
Group code
0yDDDxs25L
Also Fail Train:
Scared of Dungeons:
Group code
BoLReWSqWo
Don’t expect any of them to provide you with 24 hour groups, but you will find people in them if you hang around and watch and ask.
Ok, so your best way to start is by filling in the gaps of dungeons you haven’t done on Normal or Heroic, and watching videos of the dungeons on YouTube. You might start with Fatboss guides. Here are some others:
https://www.youtube.com/results?search_query=mythic+plus+guide+bfa
This is a great week to start! because this week’s Weekly quest is to do 4 Mythic dungeons and the Group Finder is pretty busy with people forming groups to do them. These will mostly be plain Mythics (M0) so there is no timer, and a good chance to learn.
People tend to avoid Temple of Sethraliss and Shrine of the Storm in general, but it’s not just about the dungeon, but the combination of dungeon and affix. Some affixes are really horrible in narrow corridors, for example.
Apart from affixes and time limits, Mythics are harder than Heroics, and M+ are harder than Mythics by about 10% per level, in terms of mob health and damage.
I haven’t seen Against Overwhelming Odds today, now you mention it, but I was barely in. Maybe we didn’t get it this week?