Need to clear a bed

Hey guys.. I hope this is the right section to ask this question.. I have recently carved out about 50 square feet of new yard from the forest/scrubland that most of my land is comprised of. Well, after months of hoeing and hewing, digging and chopping, I still have tons of scrub, poison ivy (luckily I am not allergic), noxious weeds... What can I use to just clear it all and start again with grass? I am not very savvy when it comes to chemicals, and I have tried Round-Up... but I swear the plants are laughing at me as I spray them... I have a few punds of Preen, but I figured that won't be much good until I can get rid of the existing weeds..

Phew... a long-winded post, but any help would be appreciated.

Thanks,

Nick