Try to set aside some time every day for your studies, ideally when your brain is at its most receptive. It's better to study for 30 minutes every day than for 3 hours once a week. If you can spare an hour a day, break it up into two or three sessions to avoid brain overload.
If you decide to study for half an hour a day for example, try to stick to that time. It's often easier to get started on an activity if you know you'll finish it after a certain time. Don't worry about completing the lesson or whatever you're working on, just try and do as much as you in the time available, and maybe you could reward yourself afterwards.
Go over each lesson several times, perhaps once in the morning, once in the evening and once several days later. Give your brain time to digest the material, but make sure the gaps between periods of study are not too long, i.e. more than a few weeks, or you will forget most of what you're trying to learn. Make sure you have got to grips with the contents of one lesson before moving on to the next.
Make sure you're comfortable with the fundamentals of the language before tackling the more advanced stuff. It will take you a while to get used to the pronunciation and orthography of a new language, but without a solid understanding of these, you'll find it very difficult to learn more.
Setting reasonable targets is a good way to motivate yourself. You could set yourself a time limit or aim for a certain level of proficiency.
Once you have got to grips with the basics of the language, learn to talk/write/read about the things that interest you. In this way you are more likely to remember to the words, phrases and grammatical constructions you encounter.
You will find that at times you're making fairly rapid progress, while at other times you seem to standing still or even going backwards. This is normal when learning a language, so don't be discouraged. If you feel like you are making little or no progress, try going over earlier lessons/exercises to see if they're easier now than when you first tried them.
You probably make the occasional mistake when speaking your native language, so making mistakes in a foreign language is nothing to worry about. What matters is getting your message across, not whether you use all the right words, inflexions, tenses, cases, etc. If you cannot think of the exact words, try using other ones. For example, say you were talking about your office and didn't know the word for photocopier - you could try describing its function: "a machine for making copies" instead. You could also try drawing pictures and/or miming if you can't think of the words.
Learn how to say things like "How do you say X in your language", "What's the word for Y?", "What's that called?", "What are they doing?", etc.
If you have a teacher or native speaker to help with your studies, ask them to point out your mistakes and to correct them. When you first start learning a new language, having every single mistake pointed out to you will be very demoralising so ask for only the more serious errors to be highlighted. When you've acquired more confidence and a degree of fluency in the language, ask for all your errors to be commented on.
If you inadvertently offend people with your mistakes it's useful to have a few stock phrases up your sleeve, such as "Forgive me, I don't speak your language very well but am trying hard to learn it." or "Why are you laughing so much?" or "What's so funny?" or "What did I say wrong?".
Find ways to make language learning fun. This could involve games, songs, stories, tongue twisters, jokes and anything else you can think of.
How to learn any language
How to Learn a Language - tips / discussion
Meetup.com - a way to meet people interested in learning languages, etc.