So, it's time to spawn in. You double-clicked the FB, and there are likely several missions there. These will have player names, town names, and numbers near them. Player name doesn't matter for the most part. Town name does - it sets your "objective" channel. If you're defending, you want to be joining a mission whose target is your town, so you'll be on the same channel as everyone else. The number beside the mission means its priority - and mission priority really just means the person who posted it has a high rank. If you take a high-priority mission, succeed, survive and despawn in the proper area, you'll gain rank points.
Sometimes, especially on an attack, the missions will have a black/green truck icon near them. This means that a Mobile Spawn, or MS, is set up. An MS is a truck that has gotten within the proper range (not too close, not too far) of its objective and deployed. Infantry can choose to spawn at the MS instead of the firebase, greatly reducing their distance to the town. This is one of the recent and very welcome additions in the 1.18 patch. Infantry from an MS come from unpredictable locations, and the MS itself is usually in some sort of cover. A good idea is NOT to run straight to town (you'll only give away the location of the mobile spawn), but to diverge. Typically, your objective is to capture a depot or civilian structure of some sort. However, it's a good idea to knock off anti-tank guns (shoot their crew, or better yet, throw a grenade) and anti-aircraft guns. Be careful, since both can roll you over.
As infantry, you're usually helpless against a tank unless you're a sapper and you know where to place the satchel charge. Don't bother wasting grenades and rifle rounds on it - you won't do any damage. It's a good idea to kill the commander of the tank if he pops the top though. A better idea is to notify those on your objective channel. Describe the location clearly, like "Sherman 400m E of town, near Caudry CP", not "Sherman" or "omg he's on me".
This brings us to the chat system. It's... limited. You can only type one line at a time. There are 4 channels that can be tuned. Two, the Objective and Mission channel, are tuned by default. Sometimes, if you didn't choose a mission, you'll only get the Origin channel - in this case just despawn and find a mission, then respawn. If you want to tune more channels, press ctrl+c to bring up the mouse cursor, select the channel list, click one of the empty boxes, and tune one of the channels. For example, on Axis, channels 18 and 28 are for the Luftwaffe North and Luftwaffe South, respectively (Allies it's 55 for RAF and 90 for French Air Force). Most channels are squad-reserved or used for attacks.
In chat you may see global messages that say "Cambrai defenders needed, OIC badger77, chan obj/41" - meaning that the channels are objective (for those that spawned with Cambrai as the target of their mission) and channel 41 (for those with a different objective - typically flyboys bringing in air support). Even if you are on the proper objective channel, it's a good idea to tune 41 and send messages on both, to keep those without the proper objective informed. Do not overwrite your objective channel.
To send a message, press F1 to tune the first channel, F2 the second, F3 the third and F4 the fourth. You can also send messages to those in the local area (40 meter radius) by pressing enter and then typing. Be careful since that lets the enemy know you are near, though they can't tell what you're saying since the chat text is garbled.