Good luck with your new venture. There are many things you can do to make sure that you and your team has best chance of success.
1. Clarity of Roles within Team:
The moment you and your friends enter into a business plan to work together, you need to make sure everyone is on the same page, everyone has clearly defined role/job-description they are responsible for in the business to make it work.
Have minimum level of productivity expected, number of hours per week etc to put into business by each member. As partnerships are hard, partnerships among friends are the hardest due to different level of expectations, with no formal discussion on it, until it is too late...
2. Services Offered - Narrower the Better:
You need to be very specific and narrow in this regard, at least in the beginning so you can demonstrate your skill and target niche clients etc...
App Development itself is very big field, I suggest to narrow it to platform e.g IOS or Android and industry e.g. Games or Data Driven or CMS etc..
Same goes with Website Development, I am in this field for 10 years, we are 5 persons team, we just do Website Development, and even then we focus mainly about PHP Projects, with no design work, just the Web Apps within narrow list of industries and we often wonder and wish to add more members for in-house graphic design etc... thing i am trying to say is that you need to very focused about skillset your team has and then focus on the best services you can provide and let the others go unless you have more team members to handle in future .. for now as narrow and niche the targeting is better it will be to convince clients and show results with your related work.
3. Target Clients Demographics
Same as services you need to be very specific and narrow to target the clients which fit in best in your situation. So you can easily track them and market to them, you have to target in the start people you can easily approach, maybe in your locality, then build up from there it help you build your portfolio as well as testimonials then you can keep expanding your target market as you grow.
You asked should you start taking projects already or first build reference project. I would suggest you can gather few mockups and projects you already have done to showcase, but instead of building something big, I suggest try to land client for the most valuable skill your team has, and try to build something for that first client even for low price than market just to get first client and project. Yes you should trying to land your client as soon as you have specified the services you are going to offer.