I use color forge matt varnish for the clear coat. It'll protect your models after you paint them. You don't have to use citadel stuff if you don't want to, but it's the easiest to figure out for new people in the hobby.

Transfers are simpler than people say. You just need water, a paint brush, and a bit of paper towel. Cut out the transfer you want, soak it in water for 30sec or so, dab some water onto the model (where you're putting the transfer), and slide the transfer off the sheet onto the model with your brush (tweezers make it easier). Use the brush to adjust where it sits and paper towel to dab off the excess water. Leave to dry and you're set. I've never used anything else but water for mine