It seems like you've gotten the damage part of it!

However, your ability to hit is modified by your Strength modifier and your proficiency bonus. With a Strength modifier of +3 and a proficiency bonus of +2 you have a bonus of +5 to hit with any melee weapon you're proficient with.

Proficiency won't be used in calculating your damage. Your damage will use your weapon's damage dice and your Strength mod. A greatsword deals 2d6 damage and then your Strength mod is added on top of that. You will deal 2d6+3 damage on an attack.

In order to kill a 7 HP goblin you will need to roll at least 4 damage on the 2d6, then your Strength mod will do the rest. It has 15 AC, so your attack roll (a d20, which is used to see if you hit with all attack rolls) needs to be at least a 10 to hit it, since your hit modifier adds 5 to the roll. As one of my DMs has always said, "Meets it beats it" when it comes to attack rolls.

Basically you need to distinguish between your raw Strength stat and your Strength modifier. Your raw Strength stat isn't used very often in calculations. However, your Strength modifier is. Your Strength modifier is a base of zero at 10 and then increases by 1 for every 2 points of Strength you have over 10. You have +3 since you have 16 Strength. Also, having less than 10 reduces this modifier.