I am not sure whether to put a comma after "so", which is at the beginning of a sentence

Some people say since "so" is a transitional word, it should have a comma after it like all the transitional words have. And some say if "so" is there for logical continuity, then no comma should be used.

The particular sentence I am writing is:

Moreover, I have completed my senior secondary studies in English and won 1st place at the regional stage of English Olympiad. So, I am confident that I will be able to complete a degree in English.

Some people say since "so" is a transitional word…

There aren't traditional words, there are traditional senses. Compare:

Mix all of these ingredients together. Next, add the milk.

I love Firefly. Next to Twin Peaks it is my favourite programme.

I've deliberately picked a different "transitional word" to hopefully make it easier to see the point if you're perhaps over-thinking so.

In the first case here, next is indeed in a transitional sense—it serves to introduce the next sentence while also stating they are in a temporal order—and the comma makes sense.

In the second case next is not doing this transitional job, but telling us something about how the topic of discussion (Firefly) relates to another thing (Twin Peaks) in the writer's opinion. We don't want to separate it from the to; what would the now separated "to Twin Peaks it is…" even mean?

So, back to so.

So's transitional sense is "used after a pause for thought to introduce a new topic, question or story." In this sense you can remove it and not really lose much meaning, though might you lose impact. If you want your sentence to mean "I am confident that …" but with so serving this introductory role, then the comma might be a good idea for that reason. (It might also be a good idea to just cut it, but that's another matter).

Another sense of so is "therefore" or "with the result that". If you want so to say that the studies you mention in the previous sentence (along, perhaps, with earlier statements) is the reason why you have this confidence, then the comma is probably a bad idea.

I imagine you want the second of these two readings, and I'd therefore recommend that you don't use a comma.

I would leave the comma out: if the two sentences were written as a single sentence, there would be no comma: "Moreover, I have completed my senior secondary studies in English and won 1st place at the regional stage of English Olympiad, so I am confident that I will be able to complete a degree in English." This is fairly formal English.

I'd reserve commas after so only when so seems to come out of nowhere: "So, are you coming or not?" A woman walks into the living room and asks, "So, how do you like my new dress?" This is hardly formal English.