You have to start somewhere. That's the bottom line when it comes to getting things done. But how do you decide just which tasks you should tackle first? Do you start with the hardest? The easiest? How do you know?
Before you consider anything else, think about where you'll be. If you're going to be in your office, you probably have many more options for just what you can accomplish than if you're going to be sitting at the airport. You just have a lot more resources at your disposal. Take a look at your tasks and get a feel for which ones you can complete where you're at.
Next, look at the available time. If you have a phone call that you know will take at least an hour, and you only have 20 minutes of time right now, you can't make that phone call. You need to do a task that doesn't take as long.
Be careful with this one. There are some tasks that may end up taking a big chunk of time. But if the fact that it's going to take a lot of time causes you to keep putting it off, you may need to break the task down. If it's going to require more than about two hours of intense effort, you should probably break it down into smaller tasks that will add up to the whole project. If you'll have a big block of time available, you can use that, but otherwise you should break it down.
Also consider urgency. Is there a task that absolutely must be done today? You can measure that by asking yourself "would I stay at work late to complete this?" or "would I stay up until I finish this?" If the answer to those questions is "yes," you need to tackle that task (or those tasks) first.
Now you finally get back to the question of "hardest first?" You've looked at your location, the time you have, and urgency of the tasks. The urgent tasks that you can do are done - so now what do you do? You probably already have a good sense of what to do next, but if you need help, opt either to do one harder task or to get several easy tasks done quickly, to get them out of the way.
If doing several easy tasks would completely clear them out for you, that may be a good option just to pare down your list. But if you know you need to work on a harder task and you're just procrastinating, then go for the harder task to get it over and done. If you know you'll feel a huge sense of relief getting the harder task done, then get it out of the way.
Use these general guidelines to help you determine what order to do your tasks in. Regardless of how you decide to work on them, however, you need to actually buckle down and do something. If you're at a total loss, pick a task and work on it until it's completed - plain and simple.