What’s a good horizontal shmup for a newbie for Steam?
Andro Dunos 2. WHEN IT'S ON SALE. Don't buy it full price. Very Gradius, R-Type, Darius etc inspired old-school style horizontal shmup.
There is also Super Hydorah which is heavily inspired by Gradius. Again only buy when it's on sale.
Ginga Force and Natsuki Chronicles. Cool shmups but a bit visually lacking maybe. Again buy on sale.
You can also get Progear on Capcom Arcade Stadium 1. Very very good CAVE developed bullet hell. You can wait for a sale but to be honest, this is one cheap to begin with. It's pretty brutal though.
Deathsmiles 1 and 2 are also available on Steam. Maybe a little hardcore and niche though. Deathsmiles 1 is really good though. Arguably the best horizontal shooter ever but the aesthetics and difficulty might be off-putting. Deathsmiles 2 is not great (awful awful visuals) but still decent enough because it's CAVE and they didn't make a single bad game.
There are the various M2 developed Darius ports with the standouts of Darius Gaiden (available in Darius Cozmic Collection Arcade) and G-Darius being two of the greatest shmups ever but very challenging. Not great for beginners but if you like how they look and have some patience then whatever go for it. Both are genuine contenders for top 10 horizontal shmups of all time.
DARIUSBURST Chronicle Saviours is also really cool but a bit hard to grasp all the various modes.
Some other modern horizontal games to consider would be Devil Engine, Drainus and Razion EX.
You will just have to look at these games and see which ones look the most interesting to you and wait for a sale to come around.
Edit - Forgot about Akai Katana (spiritual successor to Progear. CAVE developed.)