Mutual fund are broadly 2 types. Debt and Equity. Debt is like putting money in debentures. Equity is like putting money in stock market. You can be guided by Valuereresearchonline.com on which fund to select.
Once you select a fund, in internet, you can download application forms (Eg: for HDFC Fund, it is www.hdfcfund.com OR for franklin templeton, it is franklintempletonindia.com). Any doubt, contact MF office (eg: Franklin Templeton Mutual fund Delhi no. is 011-60004255). Do not go through agents, banks and brokers. They will charge commission. After filling up application forms with all details, courier it with cheque to mutual fund office. If you have time, I suggest, first time, you visit mutual fund office and complete formalities. Subsequent purchases you can send through courier.
You can apply directly as mentioned above.
No need to open DMAT account at all.