This is a common concern, and is well founded as a failure to address the issues up front can lead to a genuine loss of market competitiveness.
Firstly, it is important to understand how intellectual property laws work – at least in the absence of a written contract!
If you commission someone to do some design or development work for you, then they will at law be the “author” of that work and will own the copyright in it.
So, even though you will have the right to use those work products, so will they (and potentially in competition with you).
You can address this issue by ensuring you have a contract in place which grants you ownership of all of the intellectual property rights in the work products the individuals create for you.
However, whilst this deals with the tangible deliverables (i.e. the actual code or design materials etc. which are produced for you), it will not stop the individuals from learning about your ideas and plans.
Again, it is crucial therefore that you ensure that your contract terms cover off your key concerns, containing for example, robust confidentiality non-disclosure obligations, and perhaps also some non-compete obligations to ensure that your contractors will not start to work in competition with you, either directly or by working with another company (although note that such provisions need to be carefully drafted if they are to be legally enforceable!).