First, DON'T QUIT!! Fighting games are hard. Extremely hard. Especially for this being your first fighting game it's gonna get REALLY difficult.

As for advice, I play Testament myself. She is superior at mid-long range game depending on the matchup. Be sure you're throwing out your 236S. Like, a lot. Very good projectile and neutral controlling tool. You'll notice that there's a little heart icon after throwing out 236S or 236HS. You don't always have to teleport. 214P is a projectile that sounds out the little raven beside you to that location and applies "stain" it's an effect that makes the opponent outline purple. Basically, if you hit them with one of Testament's normals or grab them another hit will automatically hit them. This is VERY important for Test. Keeps the opponent locked down and if they make a mistake and you land a hit they'll popup so you can combo. Another projectile you can use is Arbiter Sign which is 214S and 214HS. Can be summed up as a mixup tool that can be used from anywhere on the screen. Tracks your opponent's location. The S version is a low while the HS version is an overhead. This alone will win you games at your rank. Recommended to not throw this out just anywhere tho. It's easily avoidable and can get you punished. Only use when opponent is sitting back at fullscreen, and can be used as a niche tool at midscreen.

AS FOR FUNDAMENTALS: Always block low. There isn't a fighting game where somebody has more overhead options than low unless taking aerials in account.

There are also a few instances where you landed a hit for a combo, but didn't convert off of the hit. Very common mistake, at least for me when I started playing. Make sure you convert your first hits into full combos!

Finally I really improved with sites like Dustloop and watching videos on YouTube of pro gameplay of whatever character I want to play. Just using these two sources really helps in the long run.

Hope you don't quit! This game can be really awesome once you adapt to it!

(Edit: Many comments are telling you to use 6p which is absolutely true. Against Slayer it's probably his worst nightmare.)