Even though a dictionary is one of the most important tools a person can use when learning a new language, you'd be surprised at how many people don't spend a few extra moments to choose a good one. Instead, they seem quite content to pick up the first one they see on the bookstore shelf, buy it, and then forget about it. But that kind of selection process (if you can even call it that) is likely to get you into trouble later on because the dictionary you chose might not be suitable to your purposes. If you've had trouble finding a good foreign language dictionary in the past, I recommend taking a look at the Collins Gem series -- specifically, the Latin-English, English-Latin dictionary produced by this company.
The Collins Gem Latin Dictionary is a pocket dictionary that really packs a punch. It is billed as a "mini-dictionary," and as such can almost fit into the palm of your hand. It is extremely compact and lightweight, which means it won't take up much space in your backpack or cause you any undue shoulder strain that sometimes occurs when lugging around much heavier books. But don't let its size fool you. The Collins Gem Latin Dictionary is filled with useful information that goes well beyond the definition of words.
But let's talk about definitions first, since that's obviously what you'll be buying the dictionary for to begin with. I am not an expert on the Latin language, so I can't say with any authority which words are the most common. However, I have been using The Collins Gem Latin Dictionary for several years now as a supplement to my Wheelock's Latin Grammar and other introductory texts. I have had to look up hundreds of words in my Latin dictionary, and I have yet to come across a word that wasn't included in the Collins version. In other words, even though this is a mini-dictionary, it's not likely that beginning learners will have to find another source to help them define words. This one seems to cover the basics pretty well.
One thing that is quite a bit different from full-sized dictionaries is the length of the entries. The Collins Gem Latin Dictionary is all about succinctness and doesn't drag definitions out unnecessarily. In fact, most of the time you just get one or two synonyms when using the Latin to English portion of the book. The English to Latin portion contains a bit more information, and will tell you standard details about nouns, verbs, and adjectives. In other words, you will be able to figure out how to decline nouns and adjectives and conjugate verbs from the entries in this dictionary, which is absolutely essential for any student of Latin.
Besides the definitions, this dictionary contains numerous supplemental pages intended to serve as a quick reference guide. For example, there is one page that lists ancient Roman measurement standards so that you can perform conversions in Latin. Another page lists Roman numerals in case you forget how to write some of the less common ones. There is also an extremely helpful grammar section in the middle of the dictionary that contains examples of conjugated verbs in all tenses and voices, declined nouns in all 5 declensions, a brief pronunciation guide, and even several pages to help you make sense of Latin poetry. Although this kind of information might not be necessary when you have your textbook sitting right in front of you, it does help when other reference materials aren't available. Plus, many foreign language teachers allow the use of dictionaries during exams, so this extra information could really come in handy at those times!
If you're looking for a good dictionary to help get you through your first few years of high school or college Latin, I suggest checking out The Collins Gem Latin Dictionary. It's a convenient little book that more than adequately serves its purpose.