Where to Start
The combo counter is not indicative of a real combo at all in this game. To find out if a combo is real go into Training Mode -> Team Settings and set "Knockdown" to "Type A" while also having "Guard" set to "Anti-D Guard". This will ensure the opponent blocks and techs if they can. Combos can do well over 50% HP in damage, but since Bursting cost Tension you have a defensive option after getting hit. You can burst twice if you started with a full bar so balancing defensive and offensive meter usage in a fight is important. There are burst bait combos that exist as Burst can be blocked and avoided with disjoints.
5XXX for some characters is a combo starter, while for others it's a good Modern Cancel point. Figuring out which one your character is, and finding out if their combos benefit more from Specials or Modern Cancels is an important part of character familiarity to build. Since this wiki is incomplete I recommend finding your combos in the GG2 Discord character pages or on the JP wiki using Google Translate: https://w-atwiki-jp.translate.goog/guiltygear2/?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Combos on Servants tend to be much more lenient than combos on Players so learning routes to kill them as quick as possible is just as important as learning combos against players. Also when doing combos be wary of uneven terrain as they can mess up some of the more execution heavy juggle/air combos.
Now, this is where GG2's combo system shows some cracks. In Master vs. Master combat, Guard, Bursts, and Teching keep both players from doing degenerate, ridiculous looking combos. No such restrictions exist when attacking enemy Servants, and as such, Servant Loops are born. We'll use Sol for an example here.
Almost every attack Sol has is Step Cancellable, which is very important for combo potential. On Servants, however, this is taken to an extreme. (6XX > Step Cancel Side 6XX)x3 is a true combo on all Servants, but it can't be a true infinite due to GG2's IPSInfinite Prevention System; a mechanic that causes the opponent to automatically tech, guard, or otherwise halts a combo that could otherwise be an infinite.; the enemy servants will be knocked down after a certain number of hits. If you know when this knockdown point is you can just go for your launcher into air combos at that point to maximize your damage without waiting for that servant to wake up from knockdown. Many low level servants can be killed without knocking them down saving you alot of time in the long run.
Ghosts and the Masterghost
Ghosts take far less damage from Masters then they do Servants. The two case for a Master to be hitting a Ghost. The first is that is under enemy control are if you need to get rid of the defensive buff it provides for nearby enemy units. The buff usually takes a second or two to go away after the ghost leaves enemy control. The second case for a Master to hit an enemy ghost is if there is only one adjacent enemy ghost and you are confident you can destroy the capture units between the two ghosts by blast dashing through them.
The Masterghost on the other hand should never be hit by a Master while the shield is up. Once the shield has been taken down by friendly units or by killing the enemy Master then you can consider hitting the enemy Masterghost with your Masters. High damage attacks like Sol's 8X can do alot of damage if Modern Canceled on hit and repeated. If the Tension is available and the Masterghost's health is low enough then using your Master's Overdrive can nearly one shot your opponents Masterghost. If you don't feel comfortable attacking the opponents Masterghost at all directly then you can try to rely on killing the enemy Master multiple times as the Masterghost takes damage each time it's Master is killed. A Masterghost can only die from damage taken by killing it's Master or by damage dealt directly by a Master. When the Masterghost dies it's Master (in 2v2 that Master's team) loses the game.
Most Specials take roughly 10% Tension, and Modern Cancel takes 20%. It's extremely important to understand that Modern Cancels, despite costing as much as two Specials, are often so powerful that they inflict more damage than whatever you could have done with those Specials. We'll use Sol as an example.
Sol's normal combo is 5XXX > 8X > Volcanic Viper/Bandit Revolver. With Modern Cancel, however, he can do 5XXX > 8X > MC 5XXX > dl Bandit Revolver for much more damage from of a normal hit. As well, 6XX > Volcanic Viper can become 6XX > Volcanic Viper > MC j.XX > Volcanic Viper to squeeze out damage off of a stray hit.
Infinite Prevention System
GG2's IPS is very, very prevalent in normal play, as some characters can create unblockable setups without trying very hard (On Ky, 5X > 2X > 4X isn't actually a combo, but the 4X loops behind the opponent while they're in hitstun and becomes unblockable). This is why the game's combo counter, as will be noticed during gameplay, lies frequently and doesn't actually show when a combo is true. So long as the combo counter is up, however, IPS is in effect. Using too many of one string (such as Sol's Servant Loops from earlier) will allow the opponent to tech out. Tech ChaseStepping or jumping after the opponent immediately after they tech in order to keep applying pressure. is possible though so be careful teching air combos as some characters air combos have tracking and will follow a tech with a good read. If they chose to do the tech chase and you don't tech the combo will drop.