Surfing in source games is easy to learn but difficult(and takes a lot of time) to master. Most players pick up the basics pretty quickly, however, when given proper direction. The first step in learning to surf is to learn to air strafe. To do this press one of your directional keys(primarily A and D for left and right) and slide your mouse in the direction your key is, for example if you are making a left turn, you would strafe left by sliding your mouse left and holding down A for the duration of the turn. The next step is to learn to ride the ramps. A common and simple diagram use to teach players this is d/\a. Basically what this is saying that the ramp (triangle in the middle) is used as your point of contact and you strafe against the ramp. So, if you are on the right side of the ramp, you hold your left strafe key to go into the ramp, likewise for the left side, you hold your right strafe key into the ramp. By moving your mouse towards and away from the ramp you move up and down the ramp respectively. This means that if you flick your mouse towards the ramp, you will fly up and off of it, where you can then use your air strafing to steer in the air. Now you have learned to surf, but you still should have to practice for a few hours to really get the hang of how you move on the ramps.
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There are many different 'modes' of surfing in source games. This list will explain the basic differences between the actual styles and communities of each style. This list also covers tiers, and what level of surfer you should be to attempt each tier.
Tier 1
Very early level surfers, and surfers that want to get times that have a lot of competition.
Tier 2
Early level to decent level surfers, and surfers that want to get times that have a lot of competition.
Tier 3
Decent level surfers, and surfers that want to get times that have a lot of competition.
Tier 4
Good surfers, times have less competition but still a wide range.
Tier 5
Very good surfers, and surfers that want times that are grindy, but can usually be improved greatly.
Tier 6
Expert surfers, and surfers that want to work on their smoothness/quickness of movements.

Skill Surf
The most popular and well known surf style, you are given a start zone and an end zone along with checkpoints or stages. Whoever can complete the map fastest gets ranked first with slower times falling behind in order. Community Toxicity Rating: Very Toxic
Trick Surf
Timer is not involved, however commonly done on both combat and skill surf maps, where players try to pull of the most difficult and cool looking stunts they can without losing speed or still getting to a certian destination. Community Toxicity Rating: Hardly Toxic
Combat Surf
Surfing that does not involve a timer but instead blends the modes of deathmatch or arms race with surf pitting teams of players against each other in a open style surf map that sometimes contains a jail on failure of the map, as well as many other secrets. Community Toxicity Rating: Extremely Toxic
Angle Surf
Surfing sideways, half-sideways, backwards and half-backwards by using keys other than just the traditional A and D, uses skill surf timer but is not supported on many plugins. Community Toxicity Rating: Not Very Toxic

The most important part of surfing in source games (or playing any game at all for that matter), is that you are having fun. Here are some suggestions on how to keep surfing fresh and fun.

Try angle surfing, it adds new variety and something new to learn in surfing.

Mess arounds in other modes you don't usually play.

Talk to the community of players you're with, no matter how toxic, chances are you will make a friend, and that can make surfing a lot more fun for you and the other person.

Push yourself by trying harder maps if a challenge is what you need, and speed run for perfect times on lower tiers maps if that is more your style.

If you try all of these things but you still aren't having fun with surfing, just take a break, there is no point in torturing yourself with boredom and if you branch out into other games, you could be introduced into something that peaks your interest more.

Good luck and happy surfing.

-Paddy