If you're losing weight you need to eat more food. You probably need to eat more protein.
You should pick a workout that you will be likely to complete regularly if your time is limited. A body part split is probably ideal to develop serious muscularity, but if you only make it to the gym twice a week it likely won't generate the response needed for adaptation.
Look into a full body or A/B type of routine. Make sure that you make the most out of your limited workouts, put the time and effort into the lifts that will generate the most adaptive response; the more muscles recruited the better.
This site has a search function and google is a great tool as well. Look up total body training, upper lower split, push pull split, things like that. Find one that sounds appealing and meets the above criteria and try it out.
Don't worry about supplements. Protein powder is the exception. Many people find it difficult to get enough protein to support muscle growth without using some kind of protein supplement. Grow! whey sold via the above store link is popular, tasty and cheap for the quality of the product. There are many other good options out there as well. Again, google is your friend.
You will only get out of a workout what you put into it. Lifting 'casualy' is not going to generate the adaptive response required to build muscle. You need to push yourself, set goals, meet them, and continue on. Get strong and eat enough food and you will put on muscle. There really is no other way.