Welcome!!
Typically seahorses are not kept at Reef temperatures due closed loop systems have high levels of bacteria that will cause problems with them. They also need a minimum of 30 gallons and a ton of additional filtration. They also will limit you drastically on tank mates and even more on corals. If you would like to have seahorses there is a great forum here on them!
As for corals I started with softies such as toadstools, zoas, and green star polyps. As my tank has matured I moved into LPS like duncans, hammers and acans. For you in a small tank I would recommend staying away from hammers unless you love them and pack your tank full of them. They can reach quiet a way and can kill a lot when you have a small tank and nowhere to spread things out. I have learned to love acans with all the color options and short reach.
As long as you understand that all corals defend themselves in some way and give it great planning to who is who's neighborhood you will be fine. Softies defend themselves with toxins (leathers, palythoas and zoas) or outgrow others around them (green star polyps, zoas) while LPS typically sting each other (scolly, acans, hammers) SPS some sting and some have toxins but they grow slower and can easily be taken over by softies and LPS. Try to match defenses with neighbors when finding placement. You decided what look you want in your tank and go from there.
As for fish, we have a lot more captive bred (cb) available in recent years. CB clowns are less likely to want to live in anemones, due to lack of the proper amount of slime coat. And you most likely don't want anemones in a tank that small because when it decides to move it will sting a good chunk of your corals, who can't get away from it. But on the other hand you could do a tank of only nems and they can defend themselves against each other much better than most corals.
Live aquaria is a great source for basic info. It will help rule things out and show you new possibilities. They have a whole section on nano tank fish. I am partial to clown gobies. They are peaceful, have plenty of color options and perch all over.
Most of all RESEARCH RESEARCH RESEARCH. THEN RESEARCH SOME MORE. Ask questions, yes there are some jerk on here, but they are a ton of great people here too!
Read the stickies.
Remember this is for fun not stress! Good luck! Happy reefing!