You'll need to make sure you set boundaries for your full time job otherwise they tend to claim all of your personal time, which is no easy task.
If you area really lucky and if your side project/startup involves open source code you might be able to work on the code (at least, part time) at your full time job. It's not always possible though because some companies might not even let you open source your code.
First things first, talk to your significant other about this. You will end up spending weekday evenings and weekends dedicated to a side project. If you are able to get support at home, then make sure you make the best use of your time (as I am sure you will). The trick is to figure out what milestones you want to hit in the beginning of each week, estimate the amount of time it will take, and spread out the work to each day of the week. Use a calendar or scheduler of some sort because it will keep you on track!
I quit my office job after a year of saving, and I'm about to launch the project I've been working on full-time (and plenty more) since I quit. I did have a few small gigs to keep me afloat, during that time, though.
Try not to bring the job work home. I'd usually take a 1hr nap after work so I could work on my side project with a fresh brain. Hack until 3am then sleep to reset again before work.
If you area really lucky and if your side project/startup involves open source code you might be able to work on the code (at least, part time) at your full time job. It's not always possible though because some companies might not even let you open source your code.