One of the major tasks you face when learning a language is acquiring vocabulary. The number of words you need to know depends on what you want to do with the language, and your active vocabulary (the words you can readily use in conversation and writing) is likely to be smaller than your passive vocabulary (the words you can understand).
You could try learning vocabulary systematically. This might involve learning all the words you can find related to particular topics, activities or places. For example, you could choose an activity you enjoy, such as cooking, and then learn as many relevant words as possible. You could also learn to make recipes from the country or countries where the language you’re learning is spoken, perhaps using recipes in that language.
Another technique is to learn networks of words. For example, when you learn the word for table, you could also learn words for other types of furniture (chair, sofa, etc); related verbs (eat, read, sit, etc); words for parts of a table (top, legs, etc); words for materials used to make tables (wood, metal, etc), and so on.
You could label things around your house with their names in the language you’re learning, and mentally label things elsewhere every time you see them. You could also try to describe what people around your are wearing and doing. If you can’t think of how to say something while you’re out and about, make a note and look it up later.
A way to remember words is to associate them with words or phrases in your native language that sound similar. For example, the Mandarin Chinese word for kitchen, chufang. sounds like chew fang – this might conjure up a image of a vampire chewing with his fangs in your kitchen. Don’t worry about such associations being silly or bizarre – the sillier and more bizarre they are, the easier they will be to remember.
It helps to learn words in context by reading and listening to your chosen language frequently. When you see unfamiliar words, try to guess their meanings from the context, or at least to work out whether they are verbs, nouns, adjectives or whatever. They may appear again in a different context which makes their meaning easier to guess, so don’t worry if you don’t understand them the first time.