Well Im a rookie when it comes to this plumbing thing...
I am renivating an old bathroom in my old new house, I was trying to add a sink drain to a tub drain is that possible... it seems to struggle draining a bit, I don't yet have the trap or vanity hooked up yet to the tub is draining on it own. However when it is draining water comes up the new drain... I just don't know what I need to do. Any help... and remember my plumbing knowledge starts and stops with irrigation, so please make it simple

Please explain, in detail, the connections you have made. You tied the lavatory drain into the tub drain,where and with what? And the lavatory vent, where does that go? How is the tub originally vented? Back to you, Tom