I'd spend a little time in the bookshops, library or online finding out what I liked first of all. There are tons of good books and blogs out there.
There are so many different types, styles, sizes of gardens that you need to know at least a bit of what appeals to you firstly.
Armed with that knowledge you may feel confident to take on the project yourself. Otherwise you can approach a few garden designers whose work you like and talk to them about your ideas & how to develop them. The initial legwork will save you from wasting time with designers who you aren't compatible with.
DBK100