RM Skill: Beginner
Hey there new guy. The first thing that I would do if I were you is to read the help file top to bottom. Seriously, there are some nuggets in that tome that even I don't know about all the way. But for the questions that you asked, here's some answers: Effect rating is the amount that the checked stats are decreased (increased if the target is a party member). For example, if you just want a simple damaging skill, you would put in a number for the effect rating and then check off the HP box to make it do damage.
For a variable chance of causing the poison condition, just change the success rate of the skill. But if you want to do a damaging skill that usually hits 100% but has a 10% chance on top of inducing poison, you'll have to do a bit more work to get that working. I'm pretty sure that it would involve the use of a common event but they can get pretty tricky to utilize in battles due to the events available to choose from are a bit different.
I can tell that you will have a long and confusing road ahead of you. But if you read the help file thoroughly, you will gain understanding like candy from a claw machine (the ones that let you try until you win, of course). Good luck!
I'd be glad to help anyone with RPG Maker 2003 type questions if they need assistance.