Hi Ian, welcome to the forum, your bike isn't that complicated but you don't know the history, the clutch will have stuck through standing unused for a long time. First check the oil level and top up as necessary. Then start the engine and let it run until its nice and hot (not just warm). Stop the engine, select third gear by rocking the bike back and forth as you move through the gears; then pull the clutch lever and rock the bike back and forth vigorously until the clutch frees itself.
When all is well with the clutch and while the oil is hot, drain it all out and put new oil in, you dont know how long its been in there, its a simple engine so you wont need to buy any fancy oil for it, mineral oil will do.