No matter what stage of playing you're at and what stage you are on your instrument it is important to do a warmup before you play. Similar to an athlete warming up for a marathon or a sprint we use our muscles in the same way. There are an infinite number of opinions on the perfect method of warming up, but this is mine.
First off, I like to stretch and shake out all the larger muscles in my body, playing a musical instrument can be very strenuous and will cause all kinds of injuries if there you are tense. I would start by tensing and relaxing my toes and work up my body from there. Tensing and relaxing my feet then calves then thighs etc. until I get to my face and scrunch that up and relax it. I would then shake out my feet, legs, arms and torso to make sure everything is as relaxed as possible.
Breathing is important for all instruments, however it is most important for wind players for obvious reasons. I would warm up my lungs setting a metronome to 80bpm and inhaling on the first crotchet of the bar and exhaling in time with the 2, 3 and 4 beats of the bar. This should be repeated for about 16 bars. If you feel light headed, stop, sit down and put your head between your legs if necessary.
Now for warming up the lips. I would take the mouthpiece out of the instrument and buzz the full range of what I can, trying to use the same embouchure as I would when it’s in the instrument. I would then replace the mouthpiece in the instrument and without using my tongue to start the note I would play a B flat in 1st position and sluring up to the F above it. I would then move the slide down to the next position without stopping the note and slur back down to the A in second position. I would repeat this down the slide in every position.
There are many good warmup guides including ‘Circuit Training’ by Peter Gane and ‘How trombonists do it’ by Peter Gane and Eric Crees which will add to the efficacy of your warmup.