You can probably jump in, but don't get discouraged if you get lost and confused. Kierkegaard's writing is very confusing most of the time. But as you keep reading you can find points which he makes incredibly lucid, all nestled within often confusing larger works.
Either/Or I probably wouldn't recommend as a first reading, but you've already read the Seducer's Diary, which is just a portion of book one, so you've already started reading it! Might be interesting for you to read it in context.
Fear and Trembling is always a go-to for academics to get an introduction to Kierkegaard. It's nice because it's compact, is relatively straightforward, and has a lot of discussion around it which you can easily access online. Most folks, for better or worse, have exposure to K through this work. Knight of Faith, leap of faith, etc, etc.
The Sickness Unto Death was a super life-changing book for me. About despair and not being one's true self, so this more easily fits into the larger body of existentialist works, if you're at all interested in that.
I'll leave you with some good quotes from The Sickness Unto Death
The greatest hazard of all, losing one’s self, can occur very quietly in the world, as if it were nothing at all. No other loss can occur so quietly; any other loss - an arm, a leg, five dollars, a wife, etc. - is sure to be noticed.
And thus it is precisely with the despair of finitude. In spite of the fact that a man is in despair he can perfectly well live on in the temporal, in fact all the better for it; he may be praised by men, be honored and esteemed, and pursue all the aims of temporal life. What is called worldliness is made up of just such men, who (if one may use the expression) pawn themselves to the world. They use their talents, accumulate money, carry on worldly affairs, calculate shrewdly, etc., etc., are perhaps mentioned in history, but themselves they are not; spiritually understood, they have no self, no self for whose sake they could venture everything, no self before God -- however selfish they may be for all that.