Starting Money

I'm a beginner, but I noticed when playing other more experience players that they are able to send way more creeps from the start than what I can afford based on the money I start with.

This puts me at a huge disadvantage right from the start since they can start increasing their income faster, and is why I'm always end up underwater down the road.

Do people with more experience just start with more money or what is going on?

Well more experienced Players are more experienced.
Thats their only adventage.

So the only thing you can do to change your problem ist to play better.
You have to learn a lot to master this game.

Keep trying

[Seraph07]

Everyone has the same money and gain the same income. It lies on your side how you spend it.

If you wait with your 400 credits until round 1 is about to end and send your 4 makos in the last 2 secounds + the 2 makos in round 2 you will end up with a big wave of 6 makos. Now it looks like you would have 600 credits.

Or maybe your opponents are "pushing" wich means that they send tons of low costs creeps to push their income. The first row of creeps Mercury - Manta gives you 10% income, every other row below gives you less. You gain from a demeter 9%, a Vulture 8% and for a zeus only 7%.
If you allways send the biggest creep available your opponent will outtrade you with pushing low costs creeps.