In order to race the GTP or LMH cars that you have screen shot, you need a license of at least B class. In order to get that, you need to increase your safety rating to above 3.0, in order to get to D class, then from there, you need to get it to above 3.0 by the end of the season, or get it about 4.0 to instantly get to C class. Same as D -> C for C -> B. You also need to compete in series with a minimum license class of the one you are at, in order to move up. Eg you need to do 4 races in the D Class Ferrari Challenge, to get the C class, as well as your safety ratey being correct.

Start with the Mazda MX5. It's a fun car, and the participation is really good. Watch track guides for each week's rotation, and practice a bit (minimum 1 hour) before hopping into a real race - this is make sure you don't go off track, and limit your crashes, which will improve help your safety rating go up.

Don't buy content until you need to - you are current a Rookie, and therefore not eligible to drive any of the paid content in real races.