Hi I want to re-plump my basement bathroom and add a shower and sink and move the toilet drain. How do I tie into the cast iron in the floor?

If your basement floor was not roughed in for these changes you will be doing aa lot of jack hammering. You won't be able to just "tie in" If you are serious about this project I'd get 3 bids from good plumbers and hear their ideas. This may or may not be over your head but most of the bathroom floor needs to come out, I'd do the whole room if its small.

I know there is a lot of work involved, I have already completely gutted the bathroom already
Due to water issues that I have had taken care of. I don't have a lot to spend so I was going to try it myself.

You will ned apermit so make the building inspector your friend by asking him helpful questions. Most BIs will help walk you through most of what you need. This can be a DIY project but it's a 9 on a 10 scale so I hope you have some skills, or be ready to write us tons of e-mails.