Welcome to the team! There are no stupid questions but those that you don't ask. Registers are massive machines that can be complicated. For the most part, the register will tell you everything you have to do to finish the transaction (The ones at my store have little animated pictures). Don't worry about the R/G speed scores, focus on accuracy and the speed will gradually come with it.
how do you handle a guest who wants to sign up for a redcard
You should have RedCard applications in your lane drawer. Scan the barcode on them, and select if it is a credit or debit application using the K function keys (The yellow keys) (The register will show what key is what option). Follow the register prompts from there, it will ask for ID and a check (Debit app only). If the guest is approved a temporary slip will print out, they can use this like a card until the real one shows up. If the application requires further review, something will print out saying that. You can continue to ring things up as the guest completes the application on the PIN pad/card reader.
do i have to tell a team leader anything when i'm going to clock out after my shift is over
When it is getting close to the end of your shift (usually about 5 minutes before) you should turn off your light so that you can sign off when your shift is over. Finish ringing up your guests and let the GSTL or GSA know you're leaving. Your store may have the GSTL (Guest Services Team Leader) or GSA (Guest Service Attendant) tell people when they can leave, so make sure to confirm your store's process.
and do i have to do anything with the register or money (besides sign off) when i'm done with my shift?
Nope, just sign out and head on home! A note about signing out, make sure you ALWAYS sign out any time you step away from your register to go on break or wait for guests. It will kick you out after a certain amount of time, but it is always smart to sign out whenever you step away otherwise someone could do something under your number and get you in trouble.
and do i go to any register when my shift starts or will someone tell me which lane to go to?
When you clock in, go find the GSTL or GSA and they will tell you what lane you are on. The GSTL or GSA can usually be distinguished from regular cashiers because they're the only one with a walkie-talkie, a PDA (The scanner guns or smaller version of them) of some sort, and sometimes a clipboard.
These are general guidelines, but your store might do things differently than mine. Make sure to ask someone at your store to be clear on the policy and how your store handles these sorts of things (lane assignments and clocking out). The RedCard process and not needing to do anything with your register at end of shift are global and apply anywhere.