1. Capitalism is inherently wasteful (poverty in the midst of abundant resources).
2. Capitalism can't properly address externalities (environmental damage, hazardous health effects of profitable products, and the aforementioned wastefulness).
3. Socialism could meet peoples' needs more effectively in ways capitalism cannot, and this is so because it uses resources to benefit everyone individually and collectively, whereas capitalism uses resources to benefit a few at the expense of many (and ultimately even possibly the few where things like environmental damage are concerned), and because aforementioned problems are not set aside as externalities.
There are other ways to approach it of course, but those are a few suggestions. Whatever you do, definitely define your terms (socialism, capitalism, etc.).
I'm not familiar with LD, unfortunately. I've done most of my debating in the British or Canadian Parliamentary formats. The general rules still matter, though. If you can, narrow it down to a few enumerated points under an argument with a general theme, and spend a reasonably balanced amount of time on each. You could just get up and rant and make great arguments somewhere in it, but being concise and systematic will help whoever's judging it follow along and should help you stay focused.
One last thing is to try to anticipate some common counterarguments to any points you come up with. I'm not sure how LD handles interruptions (whether you have allotted time and can't be interrupted during it or short back-and-forths might happen).
Sorry if a lot of that was generic/obvious. It's just stuff I've gotten used to repeating and hearing repeatedly since I got involved in formal university-level debating.
Good luck!
"I'm a pessimist because of intelligence, but an optimist because of will." - Antonio Gramsci
"If he did advocate revolutionary change, such advocacy could not, of course, receive constitutional protection, since it would be by definition anti-constitutional."
- J.A. MacGuigan in Roach v. Canada, 1994