You don't need to be a mechanic but don't expect to keep your hands lily-white. Here are the main things you'll need to keep an eye on.
Before any long journey check the dipstick (and check you have enough fuel, obviously!) After each day's journey, tighten the stern gland to keep the propellor shaft lubricated and sealed.
About every fortnight
Pump out the bilge if needed.
About every quarter
Oil and grease
Your engine manual will tell you how often to change the oil and what grade to use. If you don't have the manual to hand, it may be available online. If you really can't get hold of it, as a rough guide you should change the oil every three months, or more frequently if you're moving the boat most days.
If you don't change the oil then bad stuff happens: the cylinders/pistons are noisier, overheat, wear out and, if you really don't do anything at all, seize up completely.
The oil is drained by a tap near the bottom of the engine (on the oil sump), or by pumping it out. Some engines have hand pumps fitted, some use the engine's movement to pump the oil out, and others have neither, in which case you need to use a vacuum pump or other gadget of your own (you can get these from a good chandlery). Pump the oil into a container, not into the bilge, and store it.
Your engine manual should tell you how much oil to add. If you don't have the manual, use the markings on the dip stick as a guide.
If the oil's as black as a chain-smoker's lungs then you can replace it first with suitable flushing oil and then with clean oil of the correct grade.
Keep the oil fresh and the engine is likely to last you a long time.
Grease any points that your engine manual tells you to do.
Drain water from fuel
In particularly hot and particularly cold weather, condensation forms on the inside of the fuel tank. The droplets run down into the fuel to form a layer of water at the bottom. For this reason, the fuel outlet is placed above the very bottom of the tank, but if you don't drain off the water eventually it will enter the outlet and reach the engine. Not much harm comes from this but as the engine can't burn water it stops. (Just before it stops the exhaust smoke turns bright white - this is how you can tell that the problem's water in the system).
To avoid that, you need to drain off the water from time to time. Ideally there's a tap on the bottom of your fuel tank which allows this. Drain it into a container, not into the bilge.
If you're too late and the engine stops, drain off the water first, then you'll probably need to bleed the fuel system, too (remove all water and air from the pipework). The engine manual will tell you how (it takes about 20 minutes max).
About every two or three years
Black the bottom, add new anodes
After two or three years, you'll probably notice that all the black paint (bitumen) on the waterline has gone. If you do nothing, the hull will degrade very quickly. Once it starts getting pitted with rust, the rate of damage accelerates.
To avoid all, the boat should be be craned out or dry-docked for blacking. This means thoroughly blasting the bitumen from the hull, usually with a very powerful, industrial-grade pressure washer. You then leave the steel to dry and paint it at least twice with new bitumen. Try to avoid doing this in the winter as it's a job to keep the bitumen thin enough to paint on. Allowing for drying times the process takes a few days, which makes it a good time to do all your other outstanding odd jobs too. You can hire someone to black the bottom for you (often the boatyard offers this service) but it's very expensive and no-one does it thoroughly when it's not their own boat.
When blacking, check the sacrificial anodes. These are the lumps of silvery metal welded to the hull beneath the waterline; they're 'sacrificial' because they attract the electric field around the boat and corrode instead of your hull. If the anodes are more than half-wasted, add new ones. A 35-45ft boat will need about six anodes - three on each side. The boatyard will usually take care of this for you and it doesn't cost very much for them to do it. If you neglect the anodes, the hull will degrade quickly.
Change the batteries
You should expect to get a new set of batteries after two or three years. You might not need to, or you might need to get them earlier, depending mainly on how many times you've run them flat (which you should always avoid doing). Replace the whole set of domestic batteries at once - don't be tempted just to replace one at a time because the others will keep dragging down the new battery's charge to their own level.
About every five years
Have a full condition survey done, timed for early in the week when you do the blacking. This will tell you the state of the hull and will flag up other issues you might have missed and need sorting out.
Re-pack the stern tube. Get advice about how to do this.
The average annual cost of all the above regular maintenance jobs is about £550 for a 50' boat, but this doesn't include any of about 1,000 other little jobs you weren't expecting to do, like re-painting the cabin, buying a new fender, replacing a water pump, and so on... You can reckon on spending about £500 on these other jobs, although it really depends on the state of your boat.