Although mulching your beds alone will lessen your chances of developing serious weed issues by limiting the amount of light a seed receives, the only way you can be assured of weed-suppression is through use of a quality Pre-Emergent.
A Pre-emergent will keep weeds and other plants from germinating from SEED. It is not intended to kill or remove pre - existing, unwanted plants. For those, you will need to use an Herbicide such as Round-Up, or manual removal.
On the other hand, a Pre-emergent will not harm already existing flowers, trees, or shrubs that you want, OR any plants you may decide to add at a later date.