I shall arrive at Washington Dulles Airport (IAD) by Emirates flight to catch a united airlines flight to Detroit after 3.5 hours. What steps shall I have to go through? Is the connection time enough? I have US F1 visa.
The process of transferring from an international flight into the US to a domestic flight is largely the same, regardless of which US airport you go through.
The first thing you'll do is go through immigration. Last time I was in Dulles I had to stand in line for this for about 40 minutes. If your visa is in order, the actual process only takes a minute or two. If there is an issue with it, this can take quite some time. Twice I've been sent to 'the office' for follow up questions. It has never taken more than a half hour though.
Once you clear immigration, you pick up your luggage. Typically, your luggage will be on the baggage carousel by the time you clear immigration. You will always need to claim your luggage at this point. Even if it was checked through to your final destination.
You then proceed through customs with your luggage (this is why you need to claim it, since you go trough customs here, not at your final destination).
If your luggage was checked through to your final destination, there is usually a baggage re-check soon after you clear customs. They are intended for travelers such as yourself so you don't have to haul the luggage to your airline's check-in counter and baggage drop area which is possibly in another terminal (although in your case, you wont need to change terminals).
If your luggage is not checked through to your final destination, proceed to your airline's check-in counters. You can also do this even if your luggage was checked through and you missed the baggage re-check.
You may also need to claim your boarding pass from your airline's check-in counters.
Once you've dropped off your luggage and you have your boarding pass, proceed to your gate, passing through security.
When you clear customs, you will be on the ground floor, check-in counters are two floors up. See this map for more details. If you already have your boarding pass and use the baggage re-check, then you can proceed through security immediately, follow signs "TO ALL GATES".
See also this page on Dulles' website about customs and immigration.
Overall, this process will take at least 1 hour, more likely 1,5 hours. 3.5 hours should be plenty unless there is an issue with you visa.