One way that I have found, is to use a very stiff bristle brush, and sweep all areas. Pressure washers, depending on psi can destroy tarmac if old. Wash over with hose pipe, then spray or was the tarmac with a professional type moss and weed killer (you will need to get this from a licensed stockist or manufacturer outlet).
When all is dry and clean, use a sealer, spray or brush on, which you will get possibly from a builder's merchant, perhaps Wickes, Screwfix, Toolstation etc.
If there is any patching up to do, then Wickes have a cold roll product suitable for this. But be warned, use to instructions, because non-prep might result in the work not being satisfactory, especially if the area as hard usage.
If the area is 17 plus years old, then it might be time to consider re-topping.