The thing to decide before the game or during the first 30 seconds is start at your blue, your red, the other blue for invading, or the other red for invading. This can be decided by which side you start, what jungler the other team has, and CC of other champions on both sides.
Starting at your blue
The default start. Whenever you can't pick and don't know where to go, blue is the best way to go. Free cool downs is great on Trundle because of Q and faster clear times. Blue also gives all the mana you want and makes jungling safe and fast. This start also brings blue back at about 7:07-7:30, so your mid can get blue at a faster rate.
Starting at your red
Staying at your red is more risky but creates a strong ganking power at level 2. It also makes Trundle scarier than most enemy junglers with red because of how reliant Trundle is on basic attacks. Trundle can also kill the enemy jungler with red buff early on but more on that in the jungling section.
The problem with this start is that the enemy mid will mostly get blue buff faster than your mid, assuming that both mids need blue. That is what makes it risky for your team and as we all know, league of legends is a team game, so think about your team.
Going to their blue/red
This tactic is called invading. Why invade? To cripple the enemy jungler. If one buff, whether it is red or blue, is removed, you will most likely get a head of the enemy jungler and reach level 6 while he/she is level 4 or 5. This can cause snowballing if played correctly. Remember, Trundle is a dueler and even 1 level above the enemy jungler means Trundle can easily kill that jungler, assuming you don't get out played.
Most junglers start at blue but some can go to red first. This is based on who the enemy jungler is. A jungler like Amumu will normally start blue while a jungler like Mundo can start ether. No jungler has default start of red because cooldown reduction is almost too good to pass up. Ether way, invading should be based on what CC each team has, so be careful when invading. Blitzcrank and Taric are great champions to invade with but also make it hard to invade against.
If all goes well your team will get a buff and maybe even a kill or 2. However, invading is more risky than starting at your red. If one person dies, gives first blood, and your team is forced to retreat, now you or team might get snowballed.
The rest of early game
One key thing about early game for junglers is map control. To get map control, a jungler must know ether knowing where the other jungler is or being able to control buffs and dragon. Knowing where the other jungler is can be solved by wards. Keeping on ward in the enemy jungle is expected early game, this way, you have some vision. Placement of wards is just as important as having them, so always place wards in the right areas. Example: if you place a ward at a buff 1 minute after you or the jungler takes that buff, what good is it? The ward will die off before the buff re spawns. A better time to ward a buff is placing a ward 1 minute before it re spawns. Almost every time you recall to base, about 1 ward should be picked up along with other items.
One thing I like to do with my top laners that don't ward is buying and placing a pink ward at top. This way, I kill off enemy wards before ganking and ward my lane for my team. The top laner might not thank you, but you did the right thing. This works in other lanes too.
Buff control is something that isn't as easy to solve. It requires knowing the timing of when all buffs are killed, then planning on being there before the other jungler can get there. The up side is that Trundle is strong at taking buffs and running off scott free. After all, taking a buff is nothing if you end up dying with it. At about 7:20, the first blue or red on each side re spawns, use this to counter jungle
Besides map control, a jungler must know which lanes need help or ganks. If top is having problems, do your best to help that top laner. Does it mean if that laner fails in lane it is your fault for not helping as much? No, but do your best to help if possible. For Trundle, firing a pillar in lane will scare off the enemy laner, which is "help." It may not be a kill, but you don't play Trundle for ganks. Remember, Trundle is picked to counter jungler, so if you think that your team will need help before in ganks for whatever reason, Trundle may not be the best pick. Still, while in game, help out your lanes that need the most help.
Ganks are the harder for Trundle than other junglers because Trundle relays on his pillar which is not always reliable, leaving his ganks messy. Non-the-less, if Shyvana can gank and get kills than Trundle can too 3 times better. Like I said in the skill section, 70% of a gank is his pillar so if ganking, place the pillar behind the enemy champion and use frozen domain as soon as the enemy is slowed. Kite around the enemy and use basic attacks until whoever you gank runs to their tower or flashs out. Remember that getting a summoner spell and damage on that enemy is still a great gank because it gives a large advantage to your laner.
All of Trundle's ganks are about placing his W and E in a way so that both you and your laner can attack non stop while trying to kill the enemy laner. The placement of the pillar needs to be in fount of them to apply the first tick of slowing CC. If not, the enemy laner can walk around it, making the pillar on cooldown whiling doing almost nothing.
When do you place the pillar? Depends on who you are ganking with. If you have a champion with CC in your lane, placing a pillar from max range is good because the slow lets your laner do follow up CC so you can come and use basic attacks. However, if you gank top lane from behind and your laner has no CC, wait until the enemy laner tries to run away, then place a pillar to keep the chase up. So its not a "rule" for the pillar in a gank expect if Trundle's pillar is put out of place, Trundle's ganks are him running up and using a few attacks.
When ganking top or bot lane, the enemy team will try to abuse the bushes to get away. Trundle can beat this trick with a pillar in the bush that champion is in, but be careful. If the pillar is used and the enemy champion isn't in the first tick of the slow, the pillar is only used to show the enemy. So using this in bush is guess work.
: When Trundle reaches 6, you can remove armor and magic resist, making most of your damage almost true damage, and because it is a target skill, you don't need to worry about placement for that skill.
(more on junglering early game in the jungling section)