Welcome. Are you tired of joining a match of Push only to find that you're getting steamrolled by a group of level 2000s? Are you irritated at your braindead teammates who sit in a random corner 200 meters away from the objective the whole game waiting for an enemy to ambush? Do you want to stop crying yourself to sleep every night because you got blown up by a 40 mm grenade again? Because I am too, and I have a couple tips for you.

The key to a successful round of Defense is having enough players on the objective. The fewer players you have contesting, the easier it is for the enemy team to take over the objective and kill all your units. Let me give you an example:
Your team has 3 men on the objective. The enemy team attacks with 4 men. You're already at a major disadvantage. Because they have more people, the enemy team can capture the point until your units are only "blocking" the objective from being fully captured. Now all they need to do is hunt down your guys and bam, you've already lost the point and the enemy team is at 8 waves.
This is a very common occurrence in Push, and there's one big reason for that; People love to play aggressively on defense. Why sit around on point when I could stack some bodies while I lay in this corner? Now, don't get me wrong, ambushers and skirmishers are important. They can shut down lanes and bleed enemy respawn waves. However, when there is a disproportionate amount of skirmishers, the objective becomes very weak and easy to capture, if a large enough group breaks through the ambushes.
The optimal amount of skirmishers to have on, say, a 14 man team is 3. The best classes for this are sniper and gunner, as they excel at blocking lanes that the enemy team can use to advance. If those units get kills, great. If they all die, you still have 11 guys on the objective, so it's no big deal. The optimal amount of defenders standing directly on the objective is 6. The role of those other 5 is to stand at defensive locations that are right next to the objective. If you stack all of your units on the point without anyone watching the outside, it becomes very easy for an enemy with a grenade, IED, or RPG to quickly clear the point. That's why 5 of your units are holding angles outside, but can quickly rush in to the point to clear it if your 6 men inside the point die.
Not all rounds of push have 14 man teams, but the rule is the same; most of your team should be on or near the objective. Watch the doorways and chokeholds, have your commander and observer call in fire support on the main roads, and for gods sake, don't throw gaddamn smoke grenades on the point that you're trying to defend. If you do all these things, you're much more likely to have a less painful migraine at the end of the match.

Your greatest ally on Offense is smoke. Have your commander call in smoke artillery directly on the objective so that the defenders can't look out and shoot you, then throw smoke grenades to cover your path up to the objective from skirmishers and machine gunners. When you get to the point, throw as many grenades in as many rooms as you can, then get as many bodies as possible on the point and contesting.
Similar to Defense, some skirmishers can be useful in slowing down the defenders response by running towards their spawn and ambushing them as they rush to recapture the point. However, the vast majority of your team should be on the point, clearing it as quickly as possible. One issue that attackers often have is respawning and rushing onto the point without sufficient support. It's easier for defenders to fight against attackers running onto point one by one, or in groups of two, than it is to hold the point against a coordinated team of 5. Stay with your team and get on point as quickly as possible.
Arguably the most important unit for Offense is demolitions. Having C4/IEDs to throw into rooms, 40mm launchers to shoot through windows, and rocket launchers to kill technicals or helicopters is a major boon for your team. On the topic of technical trucks, in my experience they're usually death traps. I have occasionally seen them be used successfully to suppress the enemy team and shoot through walls, but the usual experience of getting in a technical truck is riding in it for 5 seconds and then getting blown up by a 40mm. I would argue it's more effective to push the point on foot.

In general, use light armor and a light carrier. Heavy armor doesn't provide much more protection than light armor, except for pistol cartridges, and significantly slows down your run speed, as well as your aiming and vaulting. It's better to be fast and avoid getting shot than to be slow and tank one extra pistol shot. Unless you're a machine gunner or love grenades, you usually won't be alive for long enough to use all 7 mags that a heavy carrier provides. When it comes to grenades, I usually bring 1 smoke grenade and 1 frag grenade/incendiary. Incendiaries and molotovs recently received a buff, so they're very viable now. If you're on offense, you might want to bring 2, or even 3 smokes, as well as a smoke launcher attached to your primary.

In conclusion, if I see you cowards sitting 40 meters away from the point, refusing to contest it as it's captured, I am going to void my bowels directly onto my gamer chair and I will cry and tell my mom on you