When you say that you're having trouble joining games, are you receiving the dreaded Mesh error? Do you get an error message saying, "Your Mesh Files Differ from the Host"? That isn't hard to fix and is a bug caused by a few extra mesh files that came with Trinity. You can find out how to fix it here:
http://TinyURL.com/Sins10
The last message in that thread might be most helpful.
On the other hand if you're talking about not being welcome at the pros' 5v5's then it's pretty much like what Greyfox said. However, often they need players and they'll let you play. Just let them know that you're new and willing to learn and follow people's advice. You might have an easier time finding newer players on the original vanilla Sins (SinsOfaSolarEmpire.exe), however the people on that server normally play comp stomps and there aren't too many 3v3s, 4v4s, or 5v5s. (Unfortunately this game just has low player counts, especially two-and-a-half years after its release.)
Anyway, I think that it's a great game and I cannot stand to play it unless I'm playing against human opponents. I hope you feel the same way and that you are able to get up to speed quickly and enjoy playing online. Try to keep a thick skin and view the games as learning experiences; getting beaten an dissecting replays is the only way to learn the game and improve. Once you learn to hold your own and become more experienced you'll probably really enjoy it.