I'd echo what Klonoa said. I have mained Sonic for 10 years and it's still much of the same stuff for him in this game. Mixing up your approaches is key. You don't wanna run in there with him and then try to make something happen out of nothing at the last minute. You'll easily get punished for that. You must have a plan for every approach. His speed is a fantastic asset for sure, but going in there mindlessly is a bad idea. Believe me, I've done it multiple times and it always leads to bad results.
His dash attack is usually a decent approach, but not something you wanna use too often. Mix in spin dashes and charges to close quickly on your opponent. It can be a great combo starter when you use it occasionally (but not too frequently). For the first time, Sonic's Homing Attack is viable. It's such an awesome way to surprise approach by tapping B twice quickly near your opponent. Using that with his speed can catch them off-guard. However, be careful when using it. It is chargeable. Once you press B a first time, Sonic will be in a ball as he rises slightly vertically. When you press B for a second time, Sonic will do the Homing Attack. It can be much stronger in this fashion, but missing will cause Sonic to lag a lot as he hits the ground. It's much better, but be on your guard.
Sonic's aerials are your best friend, especially Nair. It can be a safe approach option if you're close, especially off a short-hop (A+X). Fair can help push your opponent off the stage. Bair has great knock back and KO potential, but has some startup lag. U-air is good for juggling and is good at consecutively hitting. Don't use Dair though. It can make Sonic vulnerable, hit or no hit. I always get in trouble when I use it.
Sonic has a good grab game too. His forward and up throws have good combo potential with his aerials and specials. Back throw has good KO potential at 120-130 damage (maybe a bit less or more than that - not entirely sure). Also, don't be afraid to mix in his tilts. His forward and down tilts are safe options and can get a decent combo started. I'm trying to incorporate them more into my game as Sonic. Only use his up tilt when below a platform. It's not entirely safe to use right next to your opponent.
Best of luck using Sonic! Let the sub know how you are progressing or have any other questions regarding the blue blur. Welcome to Smash!
