I currently have 2 separate phone lines. I for phone service, and 1 is dedicated for dial-up internet. I am going to switch to DSL through my local telephone service provider. How can I make the 2 separate phones into 1? In other words, how can I have the same phone number on both lines? Any help with this would be greatly appreciated. Thanks.

Sure, Line 1 is 2 wires and line 2 is another 2 that can be disconnected and connected to line 1. This can be done outside or wher phone line goes to. You will need to use filters throughout the house. If you only had 4 conductor phone wiring and rewired jacks instead, you could have a whole house filter that will have DSL go into house on one pair, eliminating the need for filters through the house.
Doesn't hurt to remove and look at phone jacks, note what color is connected to red and green(that is line 1)
What ever color was line 2 locations will have line 2 colors connect to red and green, they need to connect to line 1 color wires connected to their red and green, Good Luck

It depends on how things were wired. If Line 1 and Line 2 are available on the same jack, the suggestions would be different if line 1 and line 2 are on two different jacks and line 1 would use a standard phone and line 2 would use a standard phone.

The suggestions also depend on where the DSL modem will go. For instance if your dial-up internet is the same place that DSL is going, then the filter in the NID is the easiest.

If your planning wireless, the wireless router should be in the center of the house.

Their can be only 5 filters in the house and a splitter outside makes a faster connection especially if the wiring is the old red black green yellow variety. Devices in the same room can share a filter/splitter as well.

ONLY the DSL modem does not get a filter. If you use a filer/splitter in the NID, then the DSL and Phone lines become separate. You could in this case wire line 1 to the phone system and line 2 to the telephone system in the same jack and use a (L1, L2 and L1+L2) 3-way connector to separate the lines. If there are two jacks, then one could be DSL and the other the telephone.

As you can see, it can be complex or simple.