I've played this game for a long time now, ksp was one of the first games i ever played, so this page is dedicated to all the tips i have.

Gravity Rings

Gravity rings are very cool, but getting a realistic spin speed is very hard so here ill tell you how to do it.
When it comes to spinning gravity rings you have two options, you either use a motor (which has limited choices for rpm) or a servo and kal9000 (both require breaking ground dlc).
In both cases you'll need to use spincalc to figure out the rpm of the ring, to do this you need the diameter which you can figure out by stacking 1 meter diameter parts drom the centre outwards.

Here in this example you can see that the ring has a radius of 9 meters, putting this into spincalc tells you that the rpm needs to be 9.96 which you can acheive using a motor so we'll have to use a servo instead.

In order to get an rpm of 9.96 you need to first set up a kal9000 controller and servo like this, make sure the controller is set to repeat and the servo dampening set to 0.
The rpm is decided by the "length" number in the top right corner but not in the way you'd think.
The "length" determines the seconds per rotation rather than rotations per minute, in order to convert we do some simple maths.
1/(RPM/60)
Which in this case, 1/(9.96/60), equals 6.
You can then go ahead and input the number (in my case 6) into the length part of the kal controller.
This will give you a gravity ring that generates 1g of centripetal acceleration, aka artificial gravity!
You may also want to set up an action group to control the speed of the ring as you will want to slowly spin it up rather than set it spinning full force straight away.

Spaceplanes

Much like gravity rings, spaceplanes are pretty cool but very difficult.
The main difficulty of a spaceplane is getting it to glide. The problem is that most of the time KSP's CoL (centre of lift) is lying to you!
In order to remedy this you need to install a mod called CorrectCoL which you can install here or through ckan which i find much simpler and easier.
Once you have this installed, your CoL should now be correct. the best place to have your CoL in a spaceplane is behind and slightly above your CoM (centre of mass) you do not want it infront (that will make it fly better backwards than forwards) and you probably want to stray away from your CoL being too high (but mostly too low) compared to your CoM.
As for launching spaceplanes, if youre doing it shuttle style it takes a lot of trial and error, you will have to do loads of tests and small changes im afraid, thats just the nature of a shifting centre of mass.
If you, however, decide to go the Dreamchaser route and launch atop a rocket you will want to put the plane in a fairing, and if you cant, put fins on the bottom of the rocket to bring the CoL down.