(Posting this here since the Sorcerer forum doesn't see as much traction, but I won't be surprised if this gets moved)

I'm a new player starting my first campaign in 2 weeks, and I think the Sorcerer has some of the best flavor and roleplay potential out of all the classes, but I've seen a buttload of discussion about why Sorcerer as a base class is something WOTC screwed up and why you should only play Wizard, and I'm feeling pretty discouraged with my choice.

Having seen some of the usual suggestions to this kind of question, I'll note that my DM has said "no" to the spell points variant which I've heard people recommend, but "yes" to subclass origin spells (specifically Shadow Magic, my origin of choice). With this in mind, what are some tips and tricks to getting the most out of my Sorcerer on and off the battlefield? Any homebrew origin spell lists you'd recommend for me to show my DM (assuming they don't get expanded spell lists in Tasha's)?

Personally, I like the Sorcerer class and have enjoyed playing it.  In fact I prefer sorcerers to wizards because they can cast spontaneously -- they do not have to decide on which of their spells to prepare in advance.  

As far as advice for a new player, I would suggest the following:

1) Pick at least 1 cantrip for ranged damage.   Fire Bolt or Ray of Frost are common choices.  This gives you a go-to option for combat without forcing you into melee. Also, if  you decide to pick more than one of these, choose different damage types since some creatures will have resistance to certain types.  Prestidigitation , Mage Hand, and Minor Illusion can also have a lot of unexpected uses. Remember that you can cast cantrips over and over without using spell slots and that they will become more powerful as you level.  

2) For first level spells, you may want to consider Chromatic Orb which lets you select the damage type again for overcoming damage resistance.  Shield or Mage Armor are also good choices and can help keep you alive in combat.

3) Don't forget your sorcery points.  You will get these starting at second level.  They are used to create extra spell slots at the level you want.

4) since you mentioned Shadow magic and have your DM's approval, you might also want to look at the spells   Shadow Blade  from  Xanathar's Guide to Everything and   Summon Shadow Spirit from the Unearthed Arcana.  The first gives you a Dexterity based weapon that deal psychic damage and the second allows you to summon a spirit that can fight for you.

This is some appreciated info. Thank you.

We'll be starting at 3rd level, where metamagic, font of magic, and other stuff starts coming in. Do you have any advice on what starting spells would be good at this level? 

Also, I've heard it recommended that it's better for Sorcerers to just convert all their spell slots into sorcery points first thing after each long rest. Do you agree?

I feel like the most fun way to play a class is to put yourself in situations where its class features are relevant. For a Shadow Sorcerer, that means *check's notes* hanging out in a Darkness bubble, surviving killing blows, harrying targets with a shadow hound, casting save spells on your harried target, teleporting, and at very high levels, being a tank. 

My takeaways:

• There appears to be some opportunities for synergies with Rogue levels (attack with advantage from a darkness bubble only you can see through; place a Shadow Hound where you want around the battlefield to create an adjacent ally). Rogues, lacking Extra Attack, enjoy using SCAG cantrips to attack, so a hybrid sorcerer/arcane trickster could be fun, if you don't let it slow your progression too much?
• A Darkness bubble does four things for you: gives you advantage on attacks against others, gives others disadvantage on attacks against you, prevents others from targeting you at all with certain "target you can see" spells and abilities, and prevents others from taking Opportunity Attacks against you when you move around them in Darkness.  If you're an Arcane Trickster, the bubble also gives you a very easy way to cast spells that enemies have Disadvantage on saves against from Rogue 9+, but that's probably more multi levels than you want to throw at it.
• This subclass has some competing uses for Sorcery Points (darkness bubbles, hound, umbral form), so you're not as much of a metamagic specialist as some other sorcerer subclasses, and you're even less free to delay Sorcerer progression by multiclassing. But, the Hound is a cheaper way to replicate one of the most expensive Metamagic options there is (3 points for 5 minutes of Hound, vs. 3 points for a single Heightened spell), so I wouldn't think of it as being a poor caster, just one that is more specialized in single-target save spells?
• The ability to teleport and phase through objects at higher levels is much more relevant for a melee combatant than it is for a ranged caster. Yet another hint that Shadow Sorcs are meant to be SCAG Cantrip  users with a Shadow Blade?

For the most part, from 6th on I really feel like its nudging towards save spells to disable/debuff/hijack opponents rather than the usual blaster focus you might see on a Sorcerer with a warlock splash. 

Hound of Ill Omens = disadvantage to all saving throws against you spell while the hound is next to them.

Hold Person / Hold Monster = paralyse target granting advantage to all hits against them, melee hits against them are auto-crits, and they're locked out of their turn. They auto fail strength and dex saves.

Mind Sliver cantrip = target has to make int save, at dis from hound, or next save is -1d4.

It's a brutally effective combo.

One thing that hasn't been discussed much is metamagic options. Here's a particular combo that pairs well if you're planning to use the melee cantrips from SCAG...

Quickened spell is already one of the more versatile Metamagic options, since it can drastically alter your options in combat. For a heavy cost of 4 Sorcery points right off the bat, on your first turn you can spend 2 sorcery points to cast darkness, 2 more points to quicken it to a bonus action, and walk right up to an opponent and hit them with a melee cantrip like Booming Blade... then simply take a step away from the enemy who can't take opportunity attacks against you in the darkness and force them to either stand there blindly or move and trigger the extra damage.

My other favorite Metamagic option is Subtle Spell, but depending on your table that might be a waste... I've seen a lot of DM's just have magic happen without anyone seeing where it came from, or allowing a stealth check to be used to see if someone can cast a spell without anyone noticing.

The other Metamagic I see grabbed most often is Twinned Spell. This can be great for doling out extra damage, but it's also really useful for buffing allies. Now with a single third level spell slot and 3 sorcery points you can cast haste on the monk and the barbarian at the same time, and suddenly you're the most popular guy at the table.