Everyone can estimate to some degree. When planning more agile like development I use a little process that has worked for me in the past, I don’t profess to be an expert on the area but they may be useful to you
A. Ask each team member to estimate their next task ahead, just focusing on the functions you need for this deliverable. Note: don't start talking about your possible plans in 12 months, they don't need to know, you have the strategic view of where its going they work on the now, the tactical aspects of the jobs, if you feel the need to perhaps agile development is the wrong way for you, hop onto a more waterfall based approach and get your architecture sorted up front to support all of the possibilities.
B. Monitor how long it actually took.
C. repeat a and b until you get the personal multiplier, this is a measure of how much they over or under estimate by, this gives you a stake in the ground.
D. optionally apply your own difficulty multiplier feeling if you have enough understanding of the problem/solution domain.
Be aware over time each person may automatically adjust their estimates to bring them in line with what they can actually achieve, this experience is usually within the project they are working. So keep a record of this.