@Devilicious dude! If you want to look good in a t shirt wear a smaller one :') if I wear a typical t shirt it can hide my frame and physique so much that people don't realise and they get a surprise when it comes off so I go for a slim fit or muscle fit to really show my physique.
Actually building muscle can be easy or difficult depending on a lot of factors. If we look at it in its simplest form, work out your muscle to stress it/ break it down and it will grow and become stronger. Muscle is created with protein, if you don't have enough protein to feed the muscle to help it repair it won't grow as well.
Roughly speaking its 1 - 1.5g of protein per 1lbs of your bodyweight.
You mostly recover over night while you're sleeping when your metabolic rate slows down, so adequate sleep is required.
You describe yourself as skinny. If you're really skinny (ectomorph) you have a fast metabolism which makes it harder to grow because your body burns calories faster. To counteract this I recommend eating more later on (not to much because this will impact sleep) but while your metabolic rate slows down it will keep the calories for longer, giving your body the fuel it needs for growth and repair. It's a cheat I've used to gain size and advise it for hard gainers.
For working out, you need progressive overload. You need to be lifting heavier or more every time. When you're a beginner you'll do this easy, the more advanced you become the harder this will be. Don't be misguided by this, you will have better days where you hit personal bests and other days where you'll struggle. You'll hit plateaus and have to shake your workout up to progress past it.
Compound moves are your bread and butter but I'm an advocate of isolation moves. These can improve compound moves by isolating the weaker parts and strengthening them and in return compound moves will reward isolation moves.
I recommend cycling your training between strength, size, and other (time under tension) endurance, myriad, German volume, just isolation etc) to maximise results.
Typically speaking training for size is hypertrophy. The aim is 3 sets of 8 reps with adequate warm up sets of 1 - 3 depending on the weight you're lifting.
More advanced _ Different body parts respond better to different rep ranges. Your triceps (10 - 12 range) will usually require more reps than your chest (8 - 10 range) but you can still see results in the 8 rep range. Legs usually respond better to higher reps (12 - 15 range) but you can vastly improve strength in the 5 - 8 range and this can boost size. This is possibly more information than you need to begin with.
You can look at full body, upper/ lower split, push/ pull split or body part split. Each training has its advantages. If you're going 4 times a week I'd avoid full body. Your intensity will dip to compensate this.
I could go on and on, bore you, confuse you, lose you. There's a lot of stuff mentioned above. There's great articles on here about building your physique, there's great YouTube channels dedicated to the stuff and a lot of guides on the internet. If you want more information about anything I've mentioned feel free to ask dude!