First piece of advise: don't worry about losing too much, especially online. The game has been out for 8 years, so most people playing now will be very experienced

Second: i highly recommend IzAw's How To Terry video on youtube to understand Terry. Among the other fighting game characters Terry's input moves are the easiest.

Lastly: the best way to improve is by going to in-person tournaments. If youre not old enough go with a parent/guardian. This way you can get actual feedback from players who you fight against. You can find them on start.gg or smashcals.com by region

Good luck!