Just chiming in, because I am building my business in this area too. I started out with pretty good plant knowledge, then worked for a bit over three years in a large, busy plant nursery. I learned a lot there. My plant knowledge is very good now, and I can rattle off basic plant care/planting recommendations, and I started doing a bit of planting plans/design. I went freelance not quite a year ago and now have a mix of regular maintenance work and planting work (I wouldn't say design because I have no design quals).
I really enjoy it and if I weren't 6.5 months pregnant I'd be drowning in work; I get more queries than I can accept and so I've paced myself.
Might just be my area and my background, but I've not found websites/online stuff necessary. All my work I've had, I've had through word of mouth and contacts from the nursery I used to work in.
My biggest tip is: always make it easy for a prospective client to say no, you're not what they are looking for.
I don't mow, trim hedges, do large-scale garden clearance (waste removal) or do hard landscaping; I don't have a spraying licence.
If they want any of that, I have a short list of people to recommend (and I always point out that getting someone who will mow the lawn once a month will be a much cheaper rate than me).
I will do planting plans, draw them a garden design and plant it for them. I explain I learned to do this at [local nursery]. I don't have a design qualification. If they want a qualified designer, I have a short list of people I can recommend; if they want references for my work, I have a list I can give them.
If it's very heavy work (eg. taking out a mature hedge or putting in big rootball hedging - upwards of 5 ft, say), I will point out I may not be the quickest person for the job and I can recommend someone else.
I've not actually had anyone take me up on any of this! But it makes me feel a ton better because then I've been totally honest, and they can take it or leave it.