This is from my own experience in dealing with farming these items with and without friends, so yeah.

1) I think that you might want to at least level to 99 first when you attempt to farm for these upgrade items called "seals". You can farm them by yourself at around 85+ with certain jobs, I would like to think, but it's better to stay safe and get to 99 on the job you're thinking about farming. It doesn't take that long now, assuming this is your first step into Abyssea, because you'll see that getting EXP is very easy in Fell Cleave/EXP PTs and it doesn't take that long to get to 99 if you're starting from 75. However, if you're farming with others, 80-85 and onward is a good place to start if you're in a group.

2) Voidwatch is separate from Abyssea; the only thing the 2 share in common is the currency both content use to acquire items, etc. You really want to just save up on stones so that you'll be able to stay in Abyssea longer when you need to. The stone is used to give you about 30 minutes of time in Abyssea up to a max of 120 minutes.When you leave Abyssea, the time you had left will be retained. Any time acquired over 120 minutes is deleted and just reverts to 120 minutes. If you log out while in Abyssea, you will still stay in the zone and your "lights will be retained", but your time will be expended as if you were still there. After your time is up, you will automatically be teleported outside the area to the maw you teleported in with. After that, if you leave without expending at least one stone in Abyssea or are forcibly ejected in the mentioned method, you cannot enter Abyssea for one hour. More time can be acquired and added to the clock by finding blue chests, or pyxis, and finding time bonuses inside them; they can also contain cruor. You usually get these blue chests to appear after defeating a monster; they must be defeated with magic. When they fall, you acquire an "azure light" increase. Azure lights increase the rate at which blue pyxis appear; the more azure lights you acquire, the more likely a blue chest will appear after a monster is felled by anyone in your PT and/or alliance. These "lights" are temporary for however long you stay in Abyssea; if you leave, they reset to a counter of 0 and must be increased again the next time you enter if you wish to utilize any of their effects. How many lights there are and what they do for you can be found on the wiki associated with Abyssea.

3) Cruor is the currency used in Abyssea and some other content; it is used for teleporting to areas in Abyssea, buying armor from NPCs in Abyssea, etc. It's basically used in place of gil, except for the instance of Battle Trophies***. You can get them easily by joining up in an EXP PT. EXP PTs usually do Dominion Ops, which is the equivalent of Grounds of Valor and Fields of Valor; you kill monsters assigned to you by an NPC (designated as a page; they can be around 5 or more "pages" for you to choose from if you want to kill harder or different monsters) and you acquire a bonus amount of EXP and cruor each time you complete it; you must report back to the NPC to obtain the reward and you must sign up for the page again if you wish to take it on. (Unlike GoV and FoV, you get cruor each time you do it regardless if a day has passed or not and it does not repeat; you must go back to the NPC, as noted before. The EXP gained also does not increase unlike GoV, unless the difficulty [denoted by stars in the game] of it is raised due to circumstances happening in the area (Martellos being drained of their HP, outposts being overrun, etc). This is a very easy and fast way to gain cruor. The NPCs assigned to this event are called Dominion Sergeants.

4) Really, you can just dive into this stuff. As an RDM, you'll probably just be healing anyway and doing some backseat black magic casting to kill monsters for azure lights if needed. Other than that, you don't really need to anything else.

Just as an extra note, you only get one stone a day if you just started out. You can increase the amount of stones you obtain if you acquire Abyssite. These key items basically imbue you with extra properties, such as gaining more time when you trade in a stone, increasing the amount of stones you can carry at one time, etc. Atmas are key items that increase your combat potential, essentially. Some give you increased STR, a 3MP/tick Refresh, increased Critical Hit rate and damage, etc. You get atmas by killing NMs when their "weakness" is hit. All NMs in Abyssea have a weakness (or really, several: there are Weapon Skill weaknesses and magic weaknesses). By striking their weaknesses, this causes them to be stunned, leaving you with an advantage. Striking true with magic causes the items they can drop to have a higher chance of dropping. This means that if they drop a seal you need, they can drop more of it and also at a higher rate. It's not always guaranteed you'll get the amount and the seal you're looking for, but it it always good to get the magic weakness done if you're seal farming. The other weaknesses are usually for atmas and just plain weakening the monster down so it can be defeated. Read up more on this and the other things I mentioned on the wiki associated with Abyssea: http://wiki.ffxiclopedia.org/wiki/Abyssea

If you have any other questions, ask and I or someone else will try to give you a quick explanation on whatever it is that you need to know.

***These are things that are acquired after defeating an NM in Abyssea. They usually drop these; these are leveled one through 5, I believe, and can be exchanged for armor provided you have the correct amount of trophies. (You can have more than one of each trophy at each of the 5 levels; the higher the level, the more powerful the NM is, usually. You get better armor presumably when trading in higher leveled trophies.) This pieces of armor are given augments that basically makes each one you get unique; they aren't given random augments until you trade for them.