Sure, one word sums it up nicely. "Paperwork". You do lots of this.
Basically, first thing you want to do is contact your unit. Call your NOSC and they will tell you who your CO and XO are. Just tell them you are a new Officer and need to be directed on who to contact and when the first drill will be for you. They are used to this and will get you all set up. Get contact info for both the XO and CO (of your own unit! This is key as they will give you the XO and CO of the NOSC instead) which the NOSC can provide. Immediately call your unit and tell them the same, you are a new Officer assigned to their unit and wanted to know how you should proceed. They will probably ask you to send them an email with all of your contact information (this is so the Chief can send you a "Welcome aboard" package and get you put on the muster list). So "pen up" an email introducing yourself. Include everything on your FINSEL letter (UIC, rank, name, etc. - they will need all of that and if you can, just scan it in and add it as an attachment as well). Include some basic information about yourself, family (if you have any), address, recruiter's name and location, and what you do for a living. This gives them an idea about you as a person and what they can expect by adding you to their unit. It also sets a friendly and open tone with your unit so they aren't having to guess about who's coming. Also this gives them an opportunity to set you up with a "sponsor"...I was assigned to another ENS that just got finished with NIOBC and DCOIC so he was a world of help.
Your NOSC should also set you up with an INDOC date. Since you are 1635, you will most likely do INDOC (paperwork, ID card, medical, and everything you need to know about being in the military and what certain things apply to you as an officer) for one day at the NOSC and then you will spend your second day off site at your unit (my 1635 unit is separate from the NOSC for security purposes). This should also be explained in the email so get this information during your first phone call. It's important to have your uniform for your first drill. Some going through INDOC didn't, but you are an officer and are expected to present yourself as one. It makes you look prepared and your unit will notice.
Also you want to do a few things during that time as well... get your NMCI account, check with security and check on your clearance progression, get NROWS access, get started with NKO courses that you need to complete, check on fitreps that might be due and see when your first BCA and Eval will be. Make sure you have all of your shots when you go to the NOSC, they can give you any you don't have up to date that day. Make sure you have all of your certs on your CAC card.
That pretty much covers it for now. Exciting stuff. Plan on doing most of this on your "off" time though. I've done most during the week up at my NOSC. Drill weekends can be hectic and busy and things tend to get "overlooked" and "lost". But if you have one on one face time with them during the week, they usually are grateful that you didn't dump it all on them on the drill weekend.
Hope this helps! Let me know if you have any questions. It moves pretty fast after you commission.
CONGRATS on your FINSEL letter! Let us know when you finally "do the deed".
-Charlie