If you really must ... It's the least recommended mode of travel IMO. Granted, roads are in pretty good condition, and Litomerice is close to the motorway E55/A17/D8 Dresden - Prague. But traffic is thick, parking in the old town is possible but not free, gas is not exactly cheap in the Czech Republic anymore - and driving along the narrow cobbled alleys is no fun. Besides, as you can see on the pictures the main square Mirove namesti would look much more beautiful without all the parked cars, no?
IMO driving is only an option if you plan to venture out to the smaller towns, villages, mountains.
- Road Trip
Labe - Elbe ticket
If you're travelling in the border region Czech Republic and Germany, say, between Litomerice and Dresden, you might purchase the Labe-Elbe ticket (in the Czech Republic) or the Elbe-Labe ticket (in Germany). It allows free travel on both sides of the border for the whole day, in Germany in the VVO transportation network (www.vvo-online.de) including trains, buses, (almost all) ferries but excluding fast trains like EC and IC/ICE. In the Czech Republic it allows free travel in the Ustecky kraj (Usti n. Labem region) on all trains (also fast trains) and buses.
There is a distinct price difference for the both versions of the ticket, depending on where you buy it: In Germany (VVO network) the price is 16 Euro for a single person, 31 Euro for groups up to five persons. If you buy in the Czech Republic the ticket is 200 CZK (about 8 Euro) for the single traveler, 400 CZK (about 16 Euro) for groups.
In Germany you can purchase the ticket at ticket machines, at ticket counters (both in advance possible, then validate it when traveling) or from bus drivers. In the Czech Republic you buy from ticket machines, ticket counters at train stations or from bus drivers / train conductors - already validated for the same day.
- Budget Travel
The main bus station is right next to the train station Litomerice-mesto. From the main square it is a five minutes walk down Dlouha street to get there. There are no bus stops in the old town proper, but along the main street that runs around it.
Buses run to many destinations: Close by towns like Terezin (concentration camp memorial site!), Lovosice, the villages around Litomerice, even to Prague directly, arriving at station Holesovice. I guess you're better off by train for longer distances, though. Tickets are available from the drivers.
- Budget Travel
When I visited Litomerice in 2012 I arrived by regional train from Usti n. Labem at the train station Litomerice-mesto. It was a pleasant train journey along the Elbe river, passing the canyon-like scenery, cute villages, vineyards around Velke Zernoseky. The train station Litomerice-mesto didn't offer any services as far as I recall, aside from a ticket counter. The plus was that the central bus station is right next to the building. Also, the old town starts right there with the nice Dlouha street leading to the main square. I don't know if this station is still in operation.
In 2013 the Czech Railways obviously reorganised the train system in this region. I took the fast train Decin - Praha to Lovosice where I changed to a regional train. From Lovosice it is just a 15 minutes ride to Litomerice, but you arrive at the station Litomerice-horny nadrazi. This means "upper station" and you guess right, it is a 10 minutes walk to the main square, mostly through not overly interesting residential areas - slightly downhill, that is, meaning that you must walk uphill on your way back. The station doesn't offer any other services than ticket sale as far as I know, it's a building that is in urgent need of renovation.
The regional trains are quite nice, clean and comfortable. Tickets are available from the conductor, too. For schedules see the website below.