I'm not sure what is meant by "sounding bad." When it comes to playing what you hear in your head, I taught my beginning students to play long tones, and gave them a "mantra": "This is a 'd,' it's fingered 13, it sounds like this, feels like this, and I want it to sound like this." Repeat ad nauseum. Do this with all the notes, and you can "hard-wire" them into your brain. Muscle memory plays a huge factor in playing the note we want. Getting the sound we want is achieved by what Bruno Walter called "impulse of will." Any problems with mechanics, however, requires a good teacher who can listen and look at you while being in the same room.
When it comes to mechanics, internet forums (even one as excellent as Trumpetmaster ) may provide some good ideas, but they are often conflicting. It's somewhat akin to asking the question: "Which is the best car?" Answers will range from "Rolls Royce," "Mercedes," "Lexus," "Ford," "Chevy," "Chrysler," etc., and there is always going to be somebody who answers "Yugo." Test-driving cars is fun, and while it is worthwhile to try out some of the concepts presented by the members of TM, the quickest fix comes from a good teacher.