To invest on AngelList, you need to be an accredited investor, and you need to have been approved to invest. You can fill out the investor application here.
Once approved, you can browse and apply to Venture Funds. Navigate to the Funds page in your Invest dashboard.
There you can view specific funds that GPs have set to discoverable and apply by clicking "Apply to invest". When submitting your application, you can include a personal note and your expected investment amount.
Applying to a fund does not guarantee acceptance. A GP still must accept you into a fund. Once accepted, you can then officially commit and sign for your investment.