To play the game you should understand what the heck the main menu buttons do first of all.
This is the main menu for the DCS World P51D selected. Yes its an older picture BUT nothings really changed so it works.

Instant Action- This option lets you start off flying in mid air with the aircraft.The missions are pre generated, and are in order from easy to hard. You also may select the season for each difficulty. Each mission is approximatley 30 min - 1 hour long.

Create Fast Mission- This option is alot like Instant Action, but instead of going straight into the fray, it lets you oultine parameters, then generates the mission how you like. It also gives you a chance to move or tweak anything once the mission is generated.

Training- Training missions for DCS, pretty self explanatory, I highly reccomend doing all of these missions, as DCS if a very, VERY, complicated game as it is nearly an exact simulation of flying a combat aircraft. DCS is not the type of game where you learn on the fly very well. Also, the P51 is a very different flying expierence from the A10 or F15, learn the tutorials, itll save you from smashing into the ground when saying "Hey yall watch this".

Mission- Lets you select from any missions you have: Downloaded, saved or created

Campaign- For DCS P51D the campaign is actually a series of challenge missions thet get incrementally harder as you go, first it starts as starting, then taking off, then eventually flying at ground level doing 180s. Its very fun for acrobatic flyers.

Multiplayer- Play with others. Stats are not tracked, clicking this will minimize the game and go to another screen.

Editor- Start creating a mission from a blank slate, lots of advanced functionality and features. Will let you literally replicate any military scenario accuatley and with ease.

Campaign Editor- Like the editor, but instead of creating single missions; this lets you link multiple missions together into a whole custom campaign. I reccomend you learn to use the Editor fluently before even looking at this.

Encyclopdia- List of every unit, sprite, object and anything else in DCS, along with stats, info and description.

Logbook- Profile for your SINGLE PLAYER profile. This profile and its stats are only affected by single player missions (including the editor). The pilot can die if you crash unless it is set to unvunrable. Your pilots stats, medals and achievents are shown here. This is not afffected at all in multiplayer. SO go crash there.

Options- Game options and stuff.

Exit- Self Explanitory

Click image if you're blind and or need better view.

1- Put flap handle and caurbourator handles (will be 3 including the flaps) all the way to the highest position. (You'll find these levers if you look at #1 on the photo and follow the arrow down)

2- Right next to #1 you'll see elevator, rudder and aileron trim. Set to 6 Deg for Rudder, 0 Deg for Aileron, and 2 Deg for Elevator, and 4 Deg if using drop tanks.

3- Check that your mixture gauge by the throttle is set to IDLE/CUTOFF, also open the throttle an inch (Look by #4 on pic to find these)

4- Put the propeller control lever to FULL (All the way to front), this is by the throttle and mixture control, has a "P" on it, sometimes the throttle blocks it.

5- If you want to shoot things turn on the gunsight thats circled by the #6 (Hit the switch and turn knobs to what you want to use).

6- Find altimiter on front dash and turn the knob so it zeros for your takeoff position.

7- Uncage the artificial horizon indicator on front dash by twisting its knob (It's Center on front)

8- Make sure parking brakes set.

9- Set fuel shutoff valve to ON (Make sure lever is UPWARDS, you can see fuel is going through by the fuel gauge indicator on bottom right of front dash)

10- Fuel Selector switch to Left wing tank (Look at #3 on the picture, see the red switch its pointing to? Make it look like that).

11- Turn on fuel booster (Its the switch that #4 is pointing to on front dash).

12- Set ignition to both (The little switch above #3 that it's pointing to).

13- Look at #2 on the picture, make sure the switches on the TOP ROW, next to it are all turned on to the upwards position.

Congrats that's the PRE STARTUP. Heres how to get that engine running.

1- Prime engine for 4 seconds if cold, 2 if already hot or running recently. (Remember where the fuel booster switch is? Go a few switches to the right, youll see it)

2- Hold starter switch (Right next to prime), while doing this, hit "M" or change the mixture switch next to throttle to "RUN" as the engine starts to spin.

3- Check that oil pressure reaches at least 50 PSI within 30 seconds. If it doesn’t, stop the engine. Idle at about 1200-1300 RPM until the oil temperature reaches 40°C and the oil pressure is steady. Check all the engine instruments. Make sure they don’t go crazy off their limits. After the engine is warmed up, idle at 1000 RPM or slightly less. Usually itll go smooth, so pretty much just if anything is spinning like crazy, somethings wrong)(If you have bad awareness, the gauges are on right side of front dash) (This final part is from the ED tips

Now your plane is started. If you listened to me at beggining you should know how to takeoff because youve doen tutorial.