Hi Yvan, when I first started, I had no idea what I was doing. I lost close to $200.00 plus in about three weeks because I built my own campaigns just by "winging it" and hoping they'd pay off. Then I was told by a Shopify Guru to use my Google AdWords credit that I had. So I went to Google AdWords and got help from one of their reps (live chat) on building three new campaigns.
I don't want to advice you on how to build campaigns or anything like that, because frankly I'm still learning myself. The best thing I can tell you is to get in touch with Google AdWords at least through their live chat help (open only on weekdays from 7am-5pm Pacific) and see if they can build new campaigns for you. The person that helped me walked me through it all, and even showed me what to do on a daily basis as far as using search terms to include/exclude keywords, etc.
I started off with $15.00/per day on three different "Google Build" campaigns. So it was $5.00/per campaign, three campaigns running per day. I wanted to promote three different products from my niche and they helped me build those three campaigns.
I'm at $20.00 per day now and I'm hoping the spike or consistency in sales has to do with letting my campaigns mature. I've kept 2 of the original Google-built campaigns and I have 2 other (different) campaigns that have been running for weeks in addition to the original ones. These 2 other campaigns were basically built off of the same exact settings as the original ones. I just played around with better keywords and keyword types, and cleaned up my ads throughout.
Also, make sure you get Google Analytics and Conversion Tracker (free with your AdWords account). It helps tremendously with seeing how visitors behave when they come to your site -- how long they stay, how many pages they look at, if they're clicking right out or not, what percentage are new and what percentage of them are returning, etc.
There's so much to learn and it'll seem overwhelming in the beginning but once you start seeing results and conversions, you'll understand better as far as how it's working the way it is and which keywords are triggering conversions, and which ones are useless and which ads to kill, etc. It'll take a few weeks to get used to, but you'll start understanding how it works.
Don't blow your money in the beginning by blindly building campaigns without their assistance. That's what I did. But yeah, definitely take advantage of every help you get from them.
Here's information on Shopify's AdWords credit. I'm not sure if you qualify for this or not yet but if not, check with Shopify and then go use it with AdWords.