Alright, so I'm someone who (before the great plague) frequently attended 40k tournaments, and play regularly with a competitive focused club.
First things first, Always go over your opponent army before the game, and always ask questions of units you are not sure of. I always make to sure to ask questions like "Is there any way X unit can run and charge", or ask what auras they have' what they do, how they can get re-rolls or modifiers etc. Gotcha moments shouldn't happen in tournaments. I normally play harlequins, so I make sure to ask my opponent if they are familiar with the army, and go over some of the main themes of the army, and what my main weapons do.
Main things to keep in mind to be aware of, is how fast a unit can move + charge, and how far weapons can shoot.
The best thing you can do is get in practice. If you can't get in practice games before an event, expect to loose games, but make notes of what didn't/didn't work, and then improve for the next time. Practice the missions, playing to the missions, and playing on a clock. Touriment games are often clocked at around 2.5 hours, so you need to practice playing fast. I recommend downloading a chess clock app, and practicing on a timer like that.
In 9th edition, events will be playing the Games Workshop Tournament pack. I'd recommend reading the missions, and the secondary missions. Some of the primary missions had additional special rules, which you don't want to be thrown off by, and each has a unique secondary.
Typically, your game plan should be to make sure you make 10vp every turn (at least) on primary, prevent your opponent from getting primary points and max secondary's. Being able to claim, and hold objectives is literally the most important thing in the game. I'd try and hold the min. number of objectives needed to get 10vp (2 or three objectives, depending on the mission), and focus on that. You don't want to spread yourself too thin. But, it depends on who/ what your playing. If you can easily get more objectives, then do so.
For secondary's, you typically want to try and go for ones you can do regardless of what your opponent is doing. Secondary's based around killing units is often a trap, as it forces your hand, and if your opponent places unit in reserves, or hides units, you will loose points. The only time I would take 'kill' secondary's is Abhor the Witch vs Grey Knights or Thousand Sons/Tzeentch, or Titan Hunter against a knights army.
I always go for Engage on all Fronts, or Line Breaker and Deploy Scramblers as my two first objectives. Scamblers your opponent cannot do anything to stop, and is an easy 10pts. Line Breaker/Engage is also very easy to score at the end of your turn.
The last objective will depend on your opponent and table. Practice makes perfect in finding out which will be easy to get for you for each game.
For a list.
You'll probably want to alter your list depending on playstyle, and how practice games go.
Troops in Rhinos will be one of the most important things you can take. Get units on objectives, and then hold them quickly. Rhinos can then protect their occupants for a shooting phase, and if they somehow survive later into the game, you can use them to charge enemy units to hold them up/deny overwatch.
I would also get two naked 5 man chaos space marine squads to just stick in strategic reserves. They'll have a small footprint, so are great for a cheeky linebreaker/engage on all fronts or to get you some of your scrambler actions done.
I'd not take any landraiders, as with all the Space Marines and Harlequins out there, a landradier will not survive more than a turn or two if you're lucky, it's too expensive and dies too fast.
I wouldn't take the skyshield, as it's massive and since you cannot deploy fortifications within 3" of any terrain on the table, you'll be hard set to actually be able to deploy it where you want to (or at all).
Hellbrutes are great and cheap, and the havocs are great (especially the autocannons and missile launchers).
Demon Princes are great too.
For Legion. I think the Creations of bile has a lot going for it. The character upgrades are really nice, and +1 move and Strength on everything is fantastic. The strats are all specialized, but are a great toolbox. Bile is pretty meh on his own, so he'll be a bit of a tax for the army.
Otherwise, Running the army as Alpha Legion is always a good bet, as -1 to be hit beyond 12" is a great tool to survive the first couple of turns.
For a basic idea of what I would run from what you have would be the following
Another character (doesn't matter)
x4 10 man CSM squads in rhinos (maybe 8 or 9 man to save points)
x2 5 man naked CSM squads
x2 havocs with autocannons
heldrake or raptors
I'd try that, and then switch things around as needed/ from what you experience while playing.
I'd run the Rhinos naked without the extra guns, as they are not there to shoot, and that'd save points.