I'm new too! Been in your shoes just a few months ago!
Guidlines: (but not rules)
1. If you're above 10000ft, increase speed. (But common sense must prevail: if you're at 11000ft, and you need to reduce to 250kt at 10000, the no point in accelerating for 5-10 seconds!
2. Below 10000, use speed brakes.
3. If levelled off between 5000 and 10000, waiting for further descent clearance, slow down to green dot, and then descend (OP DES) and accelerate to 250kts.
4. Below 4000ft, if you really need to get down quick, first level off (V/S 0), then flaps 1, flaps 2, descend, and get the brakes out.
Of course, NONE of that will matter if you have to comply with ATC or a particular STAR; in which case, just manage the whole thing, and monitor.
If you feel you're unable, then advise ATC, or request for what you need.
Hope this helps!