No matter where you begin, you'll be on I15 most of the way.
From the LA area, head east on I10 , I210 or Highway 60 to I15 North.
From Orange County, go east on Highway 91 to I15 North.
From San Diego County, head east to I15 North.
Traveling on I15 North, you'll soon pass by Ontario and thereafter you will begin a climb to the high desert. Traffic permitting, using your cruise control will ensure a steady speed and you won't be slowing down and impeding other vehicles.
Victorville, at an elevation of 2700 feet, will be the first town you come across of reasonable size with services. Next in another 30 minutes or so, is Barstow with a great many restaurants and services. Barstow is close to your half way point to Las Vegas, again depending on where you started.
After Barstow you'll begin one of many climbs to an altitude of 4000 feet. Most of these long climbs are three lanes wide with a slow lane for trucks and campers. Along the way you'll see Joshua trees which only grow above the altitude of 1400 feet. There is a long descent into Baker, which is at an altitude of around 1000 feet. After Baker, there are a series of long climbs and long descents before reaching the Nevada State Line. If you haven't stopped along the way, you'll have reached the Nevada State Line in 3 1/2 to 4 1/2 hours.
At the State Line (the town of Primm) are several resorts, casinos, restaurants and a Fashion Outlet. There's even a roller coaster for those so inclined. Some folks stay in Primm because it's cheaper and less congested. But we're on our way to Las Vegas, so we aren't stopping. Las Vegas is only another 30 miles or so up I15.
If you want to see the famous "Welcome to Fabulous Las Vegas" sign and photograph yourself in front of it, exit I15 at Russell Road and head East to Las Vegas Blvd.S, turn right and go about 1/4 mile to the sign. The sign is so popular with tourists that there is adjacent parking for easy access to the sign.
If you choose not to stop at the Las Vegas sign, you may still want to exit I15 at Russell Road, and go North on Las Vegas Blvd.S. The "Strip" begins just North of Russell Road wth Mandalay Bay being the first major resort you'll see. Traffic is often congested on Las Vegas Blvd. But it allows for a lot of rubbernecking as you make your way up the Strip.