This is for the previous game Sonic Smackdown, but a lot of it is still relevant so you can take a look at this. Below the video is the tutorial for USSD. I recommend being in game while looking at this.
Sonic Smackdown - Beginner's Guide

The Art of Battle

Defense

The Importance of Recovery

Burst to escape a combo!
T (that's the burst icon/symbol)

Defensive burst will let you escape combos but you can't burst during super moves.

You can use an offensive burst that knocks the opponent back while you are not being hit. You can use this in a combo or in neutral.
If your opponent uses a defensive burst you can use an offensive burst to counter.

Assists

Use an assist (2 Second Cooldown)

Characters have A, B and C assists, experiment and see what you like.

Change Character

Any character that has red health will heal over time if they are not the character you are currently playing.

Rings

Some moves, particularly moves that knock the opponent away makes the opponent lose rings.
Getting hit lowers red health.
If you pick up these rings you can heal red health.
The less rings you have, the more damage you take.

Ground and Wall Bounces.

These bounces are limited to one per combo. Exceeding the limit will cause the opponent to roll out of the combo. Some characters moves can ignore the ground bounce limit.

Attacks Properties

Some attacks have multiple hits.
Standing attacks are mid attacks.
Crouching attacks are low attacks.
Any jumping attack is an overhead.

Grabs bypass guard. You can't grab straight after an attack. You can stop a grab by pressing the grab input.
OTG moves take opponents off the ground.
Anti-airs are effective against opponents in the air.

Online

You select your character, colour, assists and stage in character select.
Then quickplay.

Or go to lobbies, create or join one.
You can go to character select there as well.

The game will save your team.

Team Size

Team HP and Damage changes depending on team size.

More

Ultimate Sonic Smackdown Advanced Tutorial