In Q, Who, the Federation are first properly introduced to the Borg (they had been exposed to the Borg before, but not formally introduced shall we say). When it came time a couple of years later when the Borg began their incursion into Federation space in The Best of Both Worlds Pt I, it was far sooner than the Federation had anticipated. Shelby was assigned to the Enterprise-D as a Borg expert. My question is, though, what measures had the Federation taken in preparation to deal with the Borg threat before The Best of Both Words Pt I?
Specific answers are hard to come by. Shelby:
was placed in charge of Starfleet's defensive planning for a potential Borg invasion. Under the supervision of Admiral J.P. Hanson, Shelby proved the first who was able to put the project on a productive track. Thereafter, Hanson worked extremely closely with Shelby, spearheading all efforts to develop defenses against the Borg.
With The Borg's early arrival, they were caught short:
Geordi in particular is stumped... moves to a monitor, presses panels that put up several design schemes...
GEORDI: From what I've seen, I can't believe any of your new weapons systems can be ready in less than eighteen months, Commander.
SHELBY (acknowledges) We've been projecting twenty-four.
This makes the nature of the preparation immaterial at that time. The best clue I can find to those preparations from after the fact is in the form of The USS Defiant, which
was designed for one purpose only -- to fight and defeat the Borg.
From the Defiant, and their effectiveness against the Borg sphere, we might assume that quantum torpedoes were amongst the preparations. Though The Defiant was commissioned without it, ablative armor is a reasonable guess. Apart from that, more powerful versions of existing tactical systems would be a believable guess, to wit, The Defiant's phaser banks, which, if nothing else, looked really cool.