Body Seams

I agree with jack. Try to get the fit as good as you can before the filler. If you can get the plastic to 'ooze' out of the seam you're likely in good shape. Sand down the seam and chances are you won't even need any filler.

Murray

Once you've filled it, let it dry! overnight is usually best. Then rub it down gently with wet'n'dry, start with some rough stuff & then go to a finer sheet.
When it looks smooth, spray an acrylic primer (white or grey) & leave to dry, you will usually see that it has a few flaws still. Refill, resand etc etc and eventually you'll end up with no joint line. You will then find that you have rubbed out the panel lines that need to go across the top/bottom of the fuselage, There are lots of threads on the forum for scribing if you haven't done that before.
Yes, it's worth all the trouble, there are very few aircraft that have a joint line as on the average model kit, though a couple of Messerschmitt's spring to mind!
Good Luck & welcome to the forum
Pete

The first step is to use liquid cement when glueing parts together- if applied to both parts it will soften the plastic so that the plastic will in effect fill itself(to an extent). You can then use a filler to fill in any remaining gaps.

You have to use a filler to close the gaps and then sand it smooth. There are model "puttys" out there, squadron, milliput, etc. Some people use CA glue (super glue) or automotive body filler and glazes. There are lots of different ways to do it, you just have to find the one you like.

Check out the "techniques" forum and you will find posts discussing different methods and if you have a question on something, post it out there!

I see this is you first post too snowmonster so welcome to the forum

Eric