If available, check out local startup accelerators/incubators/hubs. Many times you can set up meetings with mentors for free who can give you initial guidance and/or connections. If there aren't any available, talk to your local Chamber of Commerce. They were very helpful in pointing us in the right direction when we first started.
Start talking to people you're interested in as possible co-founders about your idea. A simple "Hey - I've got this concept. What do you think?" can start a conversation. That's what our CEO did with our other founders. Do you know anyone from your Master's program that you enjoyed working with? What skill sets are you looking to fill? Tech? Marketing? Sales? If not, LinkedIn or www.cofounderslab.com are good sites. Talk to your professors as well, for possible recommendations.
Research, research, research. Make sure you truly understand your market. Not really sure what industry you're in, but ours is a rapidly changing (and growing) one, so it's important to know what your market potential is, who your competitors are, what business model makes sense for your business, etc. TALK TO YOUR POTENTIAL USERS. This is one of the biggest lessons we've learned so far. It doesn't matter how awesome you think your product is, if no one else does, it's a lost cause.
Again, not sure what you're working on, but consider building an MVP. Or at least sketching an idea out - put it to paper. If it's a website or app, what's the general flow? If it's a tangible good, where are you getting items from? Where will you store inventory? Do you need a storefront? And on and on. You need to have your ducks in a row (as much as possible) so that you can maximize the help you receive from other startups or from possible co-founders. If you plan on outsourcing tech development, these points will be helpful as well.
I'll probably think of more, but these few bullet points kept us busy for the first month or two. So good luck!