You don't get to write off the $ amount invested in the biz, that's just your owner contribution. If you've spent the money on business expenses, you can write that off. Example:
I put $10K into my biz. I use some of that money to buy a $2K laptop. I spend $1K on website design and a website to promote my biz. I get one client, and he pays me $2K for services. I drive 100 miles to meet with client.
Here's what my schedule C looks like:
Income: $2,000
Expenses: $1,000 advertising
$54 mileage
$667 depreciation (assuming 1 full year of straight line depreciation)
Net Profit: $279 (this amount goes to line 12 on your 1040)
You'll have to fill out form 4562 for the mileage and depreciation. You can also elect section 179 and take the cost of the laptop rather than depreciating, but then you are subject to business income limits, so you'll have to carryover some of the expense to 2018, but then you'll have no net profit and therefore no taxes to pay. I'd actually opt to section 179 the laptop, but that's up to you.
I suspect I'm getting into some accounting terms you may be unfamiliar with, but just pull out the IRS instructions and go line by line to get started.