Assuming you intend to join either the Imperial or Stormcloak forces right now, you have little to worry about. Tell whichever leader, join their forces, and then request that they join the truce meeting. They will expect you to favor them during the truce talks, but that’s about all the effect this has.
If you don’t wish to join either side at this point (for whatever reason), you’re going to have to jump a few specific hoops. Talking to General Tullius is alright, but once you’ve reintroduced yourself to the man (who knows you were in the execution queue at Helgen), the game only provides you with the opportunity to tell him you’re joining up with the Imperial forces. ‘Hey I need a truce’ or ‘I want to catch a dragon’ aren’t any part of that, so you’re going to have to run out on him mid-discussion if you want to stay neutral.
Ulfric is where you really need to go- after talking to him about Helgen and mentioning that you saw each other there, his discussion moves on to a point where you can choose between joining up, talking to him about other topics, and letting him know that you intend to draw a truce. When you mention the truce, he goes on a bit about how he really doubts the Imperials will be willing, and you have three options from there- you can accept that he has issues with this (which means getting Tullius to agree before you can get Ulfric to agree, which you can’t because all you can do with Tullius is join the Imperial forces), pretend you already got Tullius to agree to show up (risky, but if it works, Ulfric will not-so-happily attend just so he can rub his legitimacy as leader of a force in the foreign man’s face), or point out that you’re holding the truce in High Hrothgar. This last is the best option, as it highlights to the Jarl that the meeting is intended to be neutral. While he’s sure that the Imperials will try to use the meeting to take as much as they can from him in exchange for the cease-fire, he’s willing to try showing up anyways.
Once Ulfric’s agreed to show up, you can head over to Solitude and talk to Tullius- this time you actually get the opportunity to tell him about the truce meeting. As with Ulfric, he’s initially reluctant to show up, but since you already persuaded Ulfric to go to the meeting you now have two options- point out that Ulfric already agreed, or explain that you’re holding the meeting in High Hrothgar. The former is your better option here- Tullius has every intent of showing up once he knows Ulfric will be there, because he’s a bit curious to see what demands the supposed High King is going to make in his endeavor to scrape up every bit of strength and legitimacy he can for the Stormcloaks. Tullius does warn you that he fully expects Ulfric to go on long rants and tangents about his cause, though, so you might want to hold your ears.