Don't worry man, we've all been there!
I've actually just come back to modeling after ten years away. My builds are leagues ahead of where they were when I dropped the hobby, no question, but I still feel like I'm finding my feet.
Apart from the already spectacular advice given above, a few things you might want to consider...
- Patience is a virtue, and doubly so when it comes to modeling. Take your time, go slow, dry fit everything you possibly can. Plan, then plan again before you glue or paint anything.
- If you haven't already, pick up some cyanoacrylate glue. You can find it at any hobby shop, and Hobby Lobby stocks it as well. I built the exact M4A3 you're building now many moons ago, and if I remember correctly, the liquid cements like Testors don't play well with those soft tracks. I think the instructions actually called for melting the tabs? Anyway, it's good to get multiple adhesives in your arsenal. A solvent (like Testors Liquid or more powerful stuff like Tenax 7R), a CA, good old white glue, etc.
- Don't let kids mess with your build! I've got two kids, and there' s a reason the workbench is in the garage!
- If you're just learning the ropes, don't start with your dream kit or your absolute favorite tank/plane/what-have-you. You'll stress yourself out trying to get it perfect. Circle toward it gradually as you gain experience. Think of the builds leading up to it as practice for the big show. Personally, my favorite plane is the P-47, and Tamiya's kit is supposed to be the gold standard. I haven't even bought one yet. I've got an old Monogram in the stash now, but I'm still working my way through other WWII aircraft first.
- Above all, hang in there and treat every mishap as a learning opportunity.
Good luck, and let us see some pics of this Sherman!