1) Time ( @ThomasP got there first). Ukraine's forces, like the Russians, are also battered, and while new classes of recruits are coming in, they too will need time to be folded into units. Tired units will need to be reconstituted, resupplied, and and brought back to battlefield snuff. Damaged vehicles will need repairs.
2) New equipment. They've likely lost quite a bit of equipment, and what isn't damaged or destroyed is probably worn by the incessant combat. In the offensive, mobility is key unless you're aiming for a grinding battle, so trucks and APCs, along with escorting vehicles. I seem to remember reading that the Ukrainians have lost about 500 tanks and other armor. Also, more artillery. Lots more, preferably self-propelled due to it being able to follow an advance efficiently.
Ideally more and better aircraft. I hope that's going on under wraps, say, in Germany, but that seems like hopeless optimism on my part.
3) Training. They will not only need training on most of the new equipment, they will need time for retraining in order to make sure that forces that have been fighting defensively can turn around and actively prosecute an offensive. They will also have to, as noted above, train and incorporate new troops into the force.
4) I haven't heard much on this score, but I bet they're going to need quite a bit of fuel.
5) A damned good plan, to maximize the capabilities of their forces.