Matt paint will dry flat, if you want to have a glossy finish, you'll have to get glossy paint, or coat the final paint job with a clear glossy coat.
For thinner, you can use Humbrol's own, or simply mineral spirits to thin the paint. Its the same stuff you use to thin enamel house paint - one small bottle will last you a long time for modelling. You can also get them at art supply stores.
To minimize brush marks, keep the paint thinned enough that it flows but not too thin so that it doesn't cover. Keep a 'wet edge', that is paint with overlapping strokes so that your wet paint will mix with the previous laid down wet paint. Otherwise, you will have a heavier coat where your paint overlaps - giving one source of brush marks. If you don't cover everything immediately when the paint is half dry, resist the urge to fix it. Let it dry completely and put in a second coat. Remember gloss enamel paint will take much longer to dry than flat paint.
Good luck.