You need to decide which battlefield 'area' you mainly want to visit, depending if you are search for a specific thing such as the grave of a relative.
If you want to visit the Somme then the area around St Quentin and Arras is the best place to go. There is a very nice site at Serucourt le Grand south of St Q which makes a good base (avoid St Q - it is an industrial dump.)
Otherwise try the municipal at Ypres. It is little more than ten minutes walk from the Menin Gate, and the town itself is both pleasant and full of history. It has free wi-fi and the facilities are clean and well maintained. The Commonwealth War Graves Commission has its N Europe HQ in Ypres - there are over 1000 CWG cemetaries in Belgium, almost one round every corner, and a significant number are within about 20 miles of the town.
Many people think of WW1 being the Somme, but in fact the Ypres Salient was far more significant. There was three battles of Ypres, the last of which is best known as Passendale at which 750,000 died in eight months - 28,000 on the first day alone. There are also three major German and a couple of French cemetaries worth visiting, and the Passendale Museum in Zonnebekke is very moving to say the least.
There is a book, Before Endeavours Fade by Rose E.B.Coombs, which is the definitive guide to WW1 battelfields, it costs about £18 from Amazon, and will tell you everything in short pieces about each area. One thing to be wary of however is that it makes very short distances seem large. For instance the aerial view of the Tunnellers Memorial to the closest mine crater looks huge where it is only 150m or so.
Look at
www.greatwar.co.uk for more info.