| Arithmetic is an e-commerce skincare company focused on helping adults with acne. We haven't done much online advertising but would like to start doing some experiments with help from someone who can optimize our advertising spend a lot better than I can myself.
Regardless of whom you're going to choose, the primary challenge of SEM is instrumentalization: using analytics, and stats to figure out what the leverage rate for different channels are. Specifically, you want to see, that for each additional dollar spent on SEM, you're getting >>$1 out in revenue, so that you can scale advertising by reinvesting profits. See  http://www.forentrepreneurs.com/startup-killer/
for more on Cost of Customer Acquisition.
A good starting point might be figuring out what your base rate on conversion is; you can get this from eg. Google Analytics dashboard. Assuming e-commerce-industry-specific 5% conversion rate, a basket size of $29.95, and a 30% margin to play with:
* Your maximum cost of customer acquisition can't exceed ~$9 (30% of $29.95) -otherwise you'll turn a loss on each sale made
* Your maximum Cost per Click can't exceed ~$0.45 (5% of $9); otherwise you'll turn a loss on each click made
You might want to plug in your own numbers into the equations above to get a feel for your business' unit economics.
(If you're not yet running A/B tests on the site, that might be a low-hanging fruit in drastically improving conversion, and profitability for both existing, and future channels)
Another good starting point is Adwords' Keyword Planner. Here, you want to search for keywords, which suggests *intent towards which your site is responsive to*, and which are below your Minimum Profitable Cost Per Click (as calculated above). Note, that the skincare product category is highly competitive: $5++ CPC is not unheard of for these searches. In the above case, the challenge is coming up with creative keywords, which strongly suggests a skincare solution, but are not yet fully capitalized.
I hope this helps in getting started with SEM. If you're stuck, or need more creative input, kindly send a PM, and we'll take it from there.