Nice, another aspiring Shulk main!
I'm a pretty good Shulk main myself, so I'll spare you some of my tips about Shulk.
Firstly, Shulk has very good range on the majority of his attacks, mainly his aerials. He also has little lag on them (barring his back air and down air), so learn how to short hop and use his aerials.
Also, try not to be predictable. Shulk has seriously poor frame data on most of his attacks, and his fastest attack (neutral A) comes out at a decent 5 frames. He's easily punishable if you whiff his attacks, so know which attacks are safe and space well with him.
NEVER use Back slash to recover, as it can't auto-snap to the ledge.
Learn his neutral B special move, Monado Arts. It is the bread and butter of any Shulk main. Know each art's advantages and disadvantages. Shulk's first monado art, Jump for instance, increases Shulk's jump height, but also lowers his defense (meaning he will take more damage).
I'll put all of the Monado Arts and its power ups below.
Jump 翔: Advantage: Higher jumps, meaning Shulk has a severely better recovery and edgeguard ability
Disadvantage: Shulk has less defense, meaning he takes more damage
Speed 疾: Advantage: Faster run, walk and air speed
Disadvantage: Shulk's attacks deal less damage, yet this flaw means it is better at combo strings, especially with his faster mobility
Shield 盾: Advantage: More weight, making Shulk the heaviest character in the game and tougher to KO. His shield is also much larger and stronger.
Disadvantage: Shulk now has inarguably the worst mobility in the game, in terms of run, walk and air speed, making him extremely easier to gimp.
Buster 斬: Advantage: Shulk now deals more damage. He also has more hitlag with his attacks, now being a threat to shields (Buster deals more damage to shields as well, making shield breaks more possible.)
Disadvantage: Shulk takes more damage as well, but not as much damage as he deals. Also he has lower knockback, but that isn't as much as a disadvantage, as it allows him to combo better.
Smash 撃: Advantage: Makes it easier to KO opponents a lot easier as it boosts the knockback of all of Shulk's moves.
Disadvantages: High risk, High reward art. Shulk weights less, making it easier for him to be KO'd as well. His attacks also do abysmal shield damage as well, making him more punishable.
There's an advanced technique called MALLC (Monado Arts Landing Lag Cancel) This is done when Shulk lands with an aerial while an art is being selected. Shulk will strike the corresponding art's pose, thus cancelling all lag of his aerial attack. This is a very good technique, reminiscent to L-Cancelling in Melee, alleviating all lag of Shulk's aerials while opening many opportunities for combos and even a shield break (with Monado Buster). Try practicing that with your Shulk. Timing is crucial, but as a user of that advanced tech, it's relatively easy to use.
We can add each other, so I can further help you better your Shulk. I can even answer more of your potential Shulk questions. My FC is below.
Starting off competitive smash, I obviously wasn't as good, considering how I struggled to find a main. Through my constant *** whooping by other good players, I finally was able to find someone I could have fun with (Shulk obviously! XD). It was frustrating not feeling any dose of improvement, but I learned to just suck it up and just play the game (This was made extremely easier playing as Shulk.) So always remember, don't worry; results will come eventually. Just focus on fixing your mistakes (saving a replay of you losing is better than saving one of you winning.) Don't waste your energy on salty biscuits (salty players who trash talk), and have fun playing as the character you like playing.
Good luck Smashing!