It is probably not gonna be easy to find low fee platforms since most of the platforms accessible in most of the country are custodial wallets, but I would probably recommend them for beginners something like Coinbase or an exchanger would be a good start to start dollar cost averaging growing your portfolio when you have a huge portfolio I could recommend putting some on non-custodial wallet or wallet that have seed phrase or private that is much safer than other platforms.

The easiest way to do it starts buying Bitcoin right now at any price since you're going to Dollar Cost averaging it doesnt really matter when you are going to buy as long as you are going to buy consistently you could accumulate Bitcoin overtime, custodial platform for sure have a lot of interest rate for investors so it might not be ideal but it can be a good start in my opinion, It would depend to how you're going to buy consistently you could buy Bitcoin weekly or every time the market is down something like that.