I'm rated on Chess.com in 1400 right now (elo). But sometimes my rating fluctuates between 1200 to 1500. As I searched on internet, I'm a beginner that is going to the intermediate level. Should I, as a beginner, practice a lot with the London system, or there is another solid opening for beginners?
It's fine to use the London system at any level. It's also fine to use other openings. It's not easy for someone to knock you out of the game immediately if you play it I suppose, thus it's more solid than some other openings with more immediately tactical lines. However don't be mistaken, every opening involves tactics, they are just pushed down the road a bit in the london.
Be reassured, there are masters who have almost exclusively played the london system as white. It is an unambitious opening that does not strive for much advantage(though some proponents often claim that because the black player likely has little knowledge of the london system, it is often the case that white get's some positional edge), and instead you must suffice with an equal middle game, maybe with some pressure. It is the opening associated with the chess players who do not want to learn much theory and skip this whole idea of an opening. The reason why it is called the london SYSTEM is because it is possible to play the same first 7 or so moves against anything, I think there may be some subtlety with move orders though. Many people know the London as the boring opening, though as mentioned in the other answers, tactics are always present no matter what opening you chose. Also, I have not investigated a great deal in this but Simon Williams supposedly has this new way of playing the London system in a very agressive manner, so it may not be the case that you cannot get something crazy from the london. However, when choosing this opening, make no pretensions that you will get to play games with tactical chaos more usually associated with gambits, e4, and etc.
I am rated 1200-1300 on blitz on chess.com. I only use London with white and I absolutely love it. I got the Ginger GM DVD from Simon Williams, and I have rewatched each chapter numerous times. There are so many nuances in the opening, and it does require intense study to understand the opening approach based on KID, QID, Sicilian, Benoni and Dutch defenses. I can crush unsuspecting opponents easily with traps in 15-20 moves, however better opponents will be more of a challenge. I do not find it boring and I love improving with it. I went from 800 to 1200 in six months.
You should pick one system that you like and matches the style of play suited to your personality. Then master that opening.
At your level there are dozens maybe hundreds of solid openings you could choose from that could work for you. Only a few would be best for you; but you need to decide that for yourself.
Playing against the London system was one of my weaknesses too. As such, I developed some line to stop some of the ideas. If the dark square bishop is developed early, black can create advantages.
[FEN "rnb1kbnr/pp3ppp/1q2p3/2pp4/3P1B2/4PN2/PPP2PPP/RN1QKB1R w KQkq - 0 1"]
This is just a starting point though, as there are several variations you need to consider. Most GMs usually exchange queens after these and Black can equalize from there with an accurate play. But for my elo, I play like this all the time.