Gas or Glow?
Let's start with definitions:

Internal combustion engine with spark plugs & ignition system that typically burns regular gasoline with a mixture of oil.

Today's RC gas engines are quite powerful, economical and reliable. Many were originally used in chain saws making parts easily available "Off the Shelf". The downside is the ignition system & a larger battery pack add weight and used in larger airframes.

Over the years a shift towards larger models has made gas powered aircraft very popular and spurred the growth and refinement of gas burning engines to replace the large displacement glow engines.

Are different terms for the exact same thing and represent the majority of models at the local flying field, although electrics planes popularity is rapidly catching up with glow powered planes.

Usually less expensive, more powerful, lighter, simple and cause less radio interference from ignition sources. But, glow fuel costs up to 10 times more than gasoline and a glow engine burns a lot of it, almost 2 1/2 times more than a gasoline engine.

It has never been easier to get into flying nitro rc airplanes with so many RTF trainers on the market, you no longer have to spend weeks building a kit first. Although some of us still do enjoy the building process as much as flying.

Points to bear in mind:

Compared to electric powered trainers, gas powered trainers are more difficult to really master. The addition of learning how to setup, start, manage fuel, glow plugs or an ignition system in itself can be daunting without help.

Consider joining a local model flying club or find another rc flyer that is willing to teach you to fly - you'll get a great out of it, make good freinds with years of experience to draw from and share your enthusiasm with others that can relate to your passion.
 
You may need items and accessories to complete your model, often the local club can lend these items so you do not have to purchase everything at once just to get started.  
 
It is always best to find expert help, going it alone very often will result in a destroyed model and a great deal of disappointment.  It is very easy for a novice to spend a great deal of time & money only to overlook a detail or make a mistake that can end the model.

If you can't find help, gas rc airplane trainers are typically of a high-wing design with noticeable dihedral, usually around the '40' or '46' size 2 stroke glow plug engine with a wingspan of around 60 inches

Nitro rc airplanes are often restricted to a club model flying field or a private field (with permission) large enough to accomodate your plane.  Only electric rc airplanes are acceptable at the local park.

Gas rc airplanes require more equipment out on the field because of the engine. And are not as simple as an electric airplane and requires a great deal more maintenance.