i will be airbrushing first time in feb for my step daughter bday cake lol any tips anyone please

AWear a mask and wear gloves. Dont try to thin down your own colors, just buy airbrush colors. Always spray somewhere else before you spray on the cake. Never work above 22 psi.

Watch the trigger.  If you pull it all the way back, your color will be very concentrated.  If you want lighter coverage, just pull it back a tiny bit.  In fact, I'd play with it first before going anywhere near the cake.  A paper towel makes a perfectly fine "canvas".

Make sure you clean the residual color from the reservoir before changing colors.  I always keep a small glass of water and paper towels nearby for this.  When you've finished using a color, dunk the front part of the airbrush into the water while pulling the trigger to get rid of the color, then spray the water you've collected from dunking onto the paper towel.  Dry everything off, then squeeze your new color into the reservoir.

Airbrush color will bleed if your buttercream (or whatever icing you've used on the cake) is too moist.  Crusting buttercream will give you the best results, IMO.

Don't try to color all in one go if you are going for a dark color.  Several coats of color, lightly sprayed, will build  up to a dark color without risking drips.

AMake a airbrushing booth. I use 3 sides of a large cardboard box. Put a piece of cardboard top and bottom. It doesn't eliminate the spray residue from escaping but it certinally helps hold back most of the over spray. A lot less cleaning up. Happy Airbrushing. I love it.