If you have a competent RC flight instructor the best plane to start with may simply be whatever your instructor feels the most comfortable with. Other than that, there are only a few important factors to keep in mind. You want something that is stable enough not to crash while you’re trying to figure out how to fly it. And you want something that you can afford.
The other major factor is size. Back in the 1980s the “nomal” trainer was a .40 sized purpose-designed trainer with a glow engine and a 4 channel radio. This was the standard until well into the 2000s, and I’m sure it predated my entry into the hobby by quite a few years as well. In recent years the standard trainer has shifted in favor of small electric planes that can be flown indoors, in the yard, or in a small park near home. I am a big enthusiast of mechanical things, and as such I do not care for electric motors. I like greasy engines that make noise and threaten bodily injury. But I agree with the current trend towards smaller models in RC flight training. My own efforts to master RC flight taught me that smaller trainers have certain advantages over larger ones.
The reason is crash inertia. If you are on the roof with an ant in one hand and a horse in the other, and you drop them both to the ground, one will break a leg and the other will simply get up and walk away unharmed. The same principle holds true for small planes and large planes. Because of the scaling effect, a small airplane has less weight per area of ground contact when it crashes than a large one does. To understand why, think of a cube one inch per side. If you drop it, one square inch will hit the ground, and one cubic inch of whatever it is made of will transmit its inertia to that side. Now double the dimension so you have a cube that’s 2 inches on each side. In this case eight cubic inches of stuff will transmit inertia to the four square inches that hit the ground. The ratio of mass to crash area has doubled. In aviation this is what is known as a nasty crash. I learned this the hard way, by crashing 20 and 40 sized trainers during my early attempts to learn how to fly. When they crashed there was always serious damage. I decided to build a Q-Tee, simply because it was less expensive to build and fly than larger airplanes, and I learned that a 16 ounce plane with a 3 foot wingspan can get stuck in a tree, cartwheel three times on its wingtips, or crash in the tall grass, with zero damage. With a small plane there just isn’t as much mass pushing on the part that contacts the ground.
Although smaller is usually better when it comes to avoiding crash damage, you also have to consider wind conditions in your local area. If you live in a very windy place, you’ll want to consider something a bit larger and heavier. A small, lightweight airplane can simply blow away like a paper bag on a windy day. If you like the idea of low crash inertia and you want to fly a small trainer anyway, you can wait for the days when it’s not so windy, or learn to get up at 5 in the morning so you can enjoy calm weather. Or you can learn to fly indoors with the new micro RC planes.
As with most things in life, RC training doesn’t have rigid rules. You really can learn to fly with just about any airplane. Contrary to popular opinion, you can teach yourself as well. But it really is a lot easier if you have competent help. So if you have a larger, heavier plane, rough field conditions, or windy weather, get a friend to assist. If you have no help, get a small, stable, slow airplane.