Hi!
In my experience, Tyranids, Orks and to a lesser degree Chaos (particularly Khorne), have a steeper learning curve than other armies, since these depend on closing to assault range as quick as possible why weathering incoming fire.
Therefore such a player of these forces needs to learn a little bit on "diversion" and "disruption".
On the diversion front.
You can get hordes of cheap infantry for your points. Use them as meat shields to get the troops that will really help you gain your objective.
For example, termagaunts are cheap and plentiful. On their own they may not be much, but imagine a detachment of two charge towards an objective with one detachment of genestealers behind. They can rush forward and soak up firepower while the genestealers can trot in and clean up afterwards.
Carnifex and similar units. They are tough and they regenerate. Send them off to enemy strong point to divert fire from precious assests and watch your opponent squirm as some will regenerate and continue to harass them.
Bio-titans, they are a horror to the enemy. Quick, resilient multi-purpose. Use them to strike deep into the enemy line. They can absorb almost anything the enemy can throw at them and regenerate as well. While you foe concentrates on them, bring up infantry and other support in its wake to take advantage of its disruption.
Dominatrix. She is one tough lady. Unlike other command units, this one can be used aggressively. Do not hesitate to do so. She can even make bio-titans look weak. A Dom leading a charge of hormaguants or genestealers is nothing to scoff at.
Disruption
You cannot fight a battle with two lines drawn up and let your opponent keep his intact and expect to win. You MUST make him divert his attention and resources elsewhere.
Use units like the lictor. It can infiltrate. Use them to harass artillery and other weak (but high yield) targets. Make the opponent fight on multiple internal fronts. Make him feel like there is nothing you cannot engage in your rear lines. The preoccupation alone will give you an edge to get your infantry within striking distance.
Harridans and gargoyles. The ultimate disruptor. Image a flight of one, two or even three harridans laden with a cargo of gargoyles that swoop down from their carrier and engage enemy units all across the front. Target artillery, vehicles or any other asset with low CAF. The harridan is a node creature that regenerates! It will rarely be shot down in one turn. Use them aggressively or even sacrifice them to pin down advance units of the enemy while you main assault comes in undisturbed.
Bio-drop pods. Nothing like the look of your opponent when you tell him that your dropping whole detachments of genestealers, hormaguants, etc in his back lines. Precious!

The goal is to use all these assets to tie down your foe and make your main body's advance safer until it is time to commit.

Primarch