Once you’ve got an idea of what you’ll mostly be using it for etc as per Atlaspine’s post, you can start building up some rough costs and see how it’s looking budget wise. You can pay for a pro design but I wouldn’t - it would use at least 2K of your budget, typically.
For instance, if you want a patio (or two) have a look at paving on a site like Marshall’s to get a feel for what you like and how much the basic materials might be as a starter for your area measurements. They can add up more than you think. Indian sandstone is cottagey, slate is smart and more modern, porcelain is even more modern… concrete moulded will be cheaper but personally I prefer less of something I really like.
Likewise if you want a shed or summerhouse, have a look online. Higher end and chunk of your budget might be Lugarde, if you’re not fussed about going all out have a look at sheds direct or similar. What colours might you paint it..
I like zoning the garden a bit, with interesting areas here and there. So I’d think about pergolas, bbq/pizza oven areas, lounge and seating areas, lighting. I’ve put in some trees for shade later down the line, these hot days are too much for me. You might want a canopy, big freestanding parasol, play area if you have kids… It will soon be a good time for garden furniture sales too.
Depending on what you like, I’d allow about 1K for planting if you have a decent sized garden. RHS website is good for details, local independent garden centres tend to have at least one resident expert who’ll enjoy talking to you.
I’m currently going through a similar process for a bit of our garden and I’ve sketched out a few ideas (even roughly to scale). I find it easier to show someone a picture than try to describe something in my head.
Best way to find a landscaper is word of mouth if possible. If they need to clear your site (eg dig out lawn or old slabs) it will be pricier because of machinery hire and skips. As always, get a written quotation with a cost breakdown. A reputable company may ask for a deposit but again as always, check credentials before (sorry this is obvious I know but bears repeating).
Ponds by the way are often more trouble than they’re worth. I’ve just got several bird baths dotted around which are used loads. Another thing I avoid from past tales from hell is decking - rats set up home under our neighbour’s at a previous house and apparently this is quite common!
Final thing on my ramble, it’s boring I know but you want decent safe fencing to frame your garden. Unfortunately it can also be expensive but a sound backdrop is a good idea before you start planting or putting up sheds.
Hope this may be of help, excited for you.