You just have to search online or make inquiries about different sites you can invest in. You just have not given us many things to work with. Are you looking to invest in Bitcoin or do you have your money but you want to buy bitcoin and hold it for years and are looking for how to buy Bitcoins?
If it is the latter, there are many apps to buy Bitcoin. You can buy at Binance, coinbase, Huobi, and many others. Just know which one is not restricted to your geographic location