You may already know this, but the bridge is only for journeys from Essex into Kent, southbound, or clockwise. In the other direction, into Essex from Kent, northbound, anti-clockwise, you will use the tunnel.
Traffic on the bridge can be affected by traffic (frequently) and weather (infrequently). A few days ago, when we had high winds, there were delays of several hours for the bridge.
If you read the previously noted FAQs you will see that you can either prepay for your crossing or pay after the crossing, by midnight of the following day. Pay by phone is also easy, but I use an account. If this will be your only use of the Dartford Crossing an account is probably not worth the effort. It gives me a discount on the toll, and I buy £10 worth at a time and it just comes off the account automatically when I make a crossing, and then tops up automatically.
Bill, if you give a little detail about the actual drive, there are a number of locals on here who can give a few hints about the different roads.
I can give hints about driving in Essex and Kent (and London and elsewhere) because I do it a lot.
One thing to bear in mind is I think you said before that you would be here in May? If so, beware trying the Dartford Crossing (or anything on the M25) on or near either of the May Bank Holidays. If you do try it you will understand. The same goes for rush hour which starts around 6 and goes to around 10, and in the afternoons from around 3 or 4 to about 7. That's Monday to Friday, but Saturday can be busy (to a lesser degree) all day.
Saturdays are particularly bad at Blue Water and Lakeside shopping centres so beware the A2. When I go to the south Kent area I always go beyond the A2 / M2 and stay on the M25 to the M20 which is only very little further but faster......