Panel descriptions are tricky. There's a variety of ways to write them, from the notoriously dense descriptions of Alan Moore to the more straight forward style of Garth Ennis. Whatever way you choose to go, there's a few universal rules to keep in mind.
First, someone is drawing this. There are plenty of artists who will draw whatever you write to the letter, so you can only imagine their reaction if you send them something like this:
Panel One: Double page splash! The Gladiator has entered the arena. There's about 50 other warriors in the arena already, and it's packed with thousands of people in the stands.
Now for you that might seem like a perfectly acceptable panel, but think about what you're asking for. That sentence took you less than a minute to type, but to get that level of detail it would take an artist almost a week to draw, if not more. That's not to say that you should never ask for something like that, but you should make sure you understand the amount of work you're asking an artist to put in, and if the scene is really adding anything to the book or is just something that looks cool.
In the same vein, make sure you give your artist something concrete to go on. Again, each panel doesn't have to be a dissertation, but make sure you're telling your artist exactly what is happening. Describe the setting, the characters, the mood, etc. For example, compare:
Panel One: Commissioner Gordon has taken a seat at his desk. He looks worried. Bullock on the other hand, is grinning as he pulls out a pack of cigarettes from his coat.
Panel One: Commissioner Gordon sits down. Bullock starts to smoke.
Both of those are describing the same thing, but one is giving your artist more context and mood, while the other is just general (and boring) information.
Another thing your artist will appreciate is if you call your shots. Meaning if your character is going to hit someone with a bottle on panel five, you should set it up in panel one or two. Otherwise there's a very good chance your artist will get to panel five and say "wait, where did they get a bottle from?" It makes everyone's job a lot easier when you very clearly indicate key plot points ahead of time, so if you know something or someone is going to play an important role, give your artist plenty of notice. Same thing goes for location and time of day. If the scene is supposed to take place at night, make sure your artist knows this or else you might get what is meant to be a dark, gritty street scene set in broad daylight.
If you're just starting out, the key thing to keep in mind is the amount of action that can be done in a single panel. Spider-Man can't swing into a room, land, and walk over to the closet door and open it, all in one panel. You'll sometimes see panels with multiple or after images of a character jumping across rooftops, but that's a technique that shouldn't be overused and is really only effective in certain situations. Instead think of what is essential and cut around everything else. So that above panel with Spider-Man could be; panel one he swings in, panel two he lands, panel three he walks over to the closet. You can really cut out whatever you want, as long as it's a coherent sequence. For instance, Spider-Man shouldn't swing into a room and then in the next panel open a closet door. There needs to be something in between showing us how he got there.
Lastly I would say, if you can help it, at all costs don't cop out of a fight/action scene. I'll admit I've written a script that basically said pages 5-9, the race continues. That was bad and wrong, although me and the artist did talk out the basic beats of the sequence so they could lay it all out. I did it because I feel like artists know better than I do how to work out a fight scene or in this case street race. I still feel this way, but I do my best to add as much context and input as possible. Reason being, it's the least you can do. If someone is going to take the time to draw it, you should take your time to write it. I'll often start with a note that says, this is how I see things going, but feel free to add or subtract as you see fit. It's also just a bad habit to get into. Certain shortcuts are acceptable and probably smart in order to hit deadlines, but never take the easy way out just because you can. Your artist will definitely appreciate having more to go on than, "Page 4-10, big fight scene! Have fun!"
That's it for now. Take some time to look at your scripts and see if you can tweak them with some of the tips listed above. Sometimes all it takes is a slight rewording or maybe a few more details to get a panel right. All in all, the more thought you put in on your end, the easier it will be for your artist. And a happy artist is something you definitely want. Next time I'll talk a bit about panel flow and pacing but for now, go make some comics!