My suggestions are based on working as a bar back and as a seafood wholesaler serving both food trailers and brick & mortar restaurants.
You don't say it directly, but I'll assume you're interested in opening a specialty food trailer (as opposed to the snack and sandwich food trucks that serve constructions sites and such.) My comments will work off that assumption.
* First, if you haven't already, work for awhile in the business end of an established restaurant - preparing a budget, keeping inventory, ordering supplies (everything from napkins to cooking oil to fresh vegetables to bottled soda) and paying invoices, building relationships with vendors, managing cash flow, etc. The more experience you have estimating and purchasing weekly supplies the less chance you'll run out of something or have perishable goods go stale. It will also help you stay within your budget. Also, keep in mind that storage space in a food trailer is limited, so accurate purchasing is critical.
* The other respondents have mentioned doing market research before opening - potential customer base, prevailing culinary tastes and preferences in your area, how many other folks offer food similar to yours, etc. I agree, but add this observation: The food trucks that succeed have both a limited and unique menu. Think Asian-Creole fusion rice bowl. Or breakfast tacos using a seasoning mix your grandmother created that's built around an ingredient only found in her hometown. For either of these, you can come up with a dozen variations for customers to choose from that are still only built from the same half dozen ingredients. Having a unique menu or offering a twist on a common staple (grilled chicken with a spicy sauce from Argentina) will help your food truck or trailer stand out from all the others. Having a limited menu will make it easier to serve customers fast during a rush and simplify inventory and purchasing.
* Many of the food trucks and trailers in Austin operate in outdoor food courts - five or six trailers, with a good mix of food styles (say, Mediterranean, Asian, specialty hot dogs, pastries and softserve ices). The advantage of this is that greater variety will draw a larger potential customer group day to day, and open the potential for impulse buys ("I came for ice cream, but those tacos look really good.") They also usually have tables for customers to sit and eat. From a business standpoint, the trailers can operate on the property owner's zoning and food service permits, instead of having to do it themselves. It's worth checking out in your city - the more you can piggyback on permitting and regulatory work that other folks have already done, the better.
* After you've been in business for a few months and have a good sense of your weekly inventory needs, you may find it worthwhile to make use of a commercial kitchen to store some of your frozen or perishable stock or do advance food prep (we used one for storing frozen shrimp and as a place to cut & weigh fillets before making restaurant deliveries.)
* Get to know your health inspectors. If you are on good terms with them, and have a solid track record of clean inspections, you may be able to get a bit of grace time to rectify a minor slip up, instead of getting a bad mark. They can also tell you all the rules for offsite storage and transport of perishables and nonperishable supplies, the correct way to lay out samples or finger food at your truck, etc. And since you never know when they will turn up, it's wise to keep extra copies of your licenses and permits in your food truck, in any other vehicle you use to transport perishable goods, and at your commercial kitchen.
* Lastly, advertise advertise advertise. Word of mouth is good, but when you're starting out, set aside ample room in your budget for print advertising, flyers to post up around colleges and universities, push cards to set out next to the napkins at your truck, etc. Make sure you have an online presence with hours, location, menu, contact info, etc. Design a simple web ad and keep the image on your phone so you can share it on a moment's notice. Ask other truck owners where and how they advertise - the lower the cost and higher the circulation, the better. Finally, get to know the food writers in your city and invite them to check out your offerings - there are a lot of freelance "Best Late Night Food Trucks in Omaha" blog writers out there who are always looking for a fresh angle to sell copy, and will give you a write-up in exchange for a comped meal.
Best of luck to you.