Hey there. Here is an online painter for Tyranids which may help you.
http://www.fallensunasylum.org/nids/onlinepainter.html
I've painted alot of Tyranids myself and I'll give you the basics about painting them. Tyranids come in two broad sections, the carapace or shell which you see on the back and on the head as well as on the legs etc. The other important part is the skin or exoskeleton which is the rest of it's body. It is easier to divide your Tyranids into these two parts. Choose two colours you like and that you think will work well together and look good on your bugs.
There are two ways to paint carapace. One is dark. You paint it a dark colour which will need very little shading. You then highlight the edges. An example would be dark green. Once painted that colour you will only need a little shading with black and then a little highlighting. The other way is a light colour like a bone shade. You would need more shading but because the colour is light it needs very little in the way of highlighting.
The skin is best done being painted a colour you like. Then drybrush it with a slightly lighter colour. After that give it a wash of a colour that works, blue wash with blue paint, green wash with green paint etc. You can then add some more highlights to enhance the look of your bugs with a quick light drybrush.
Tyranids can be fun because you can invent your own Hive fleet and give them whatever colours you want. Other bits are some pink for the tongues or green for the tongues if you go for red Nids (red is a popular Tyranid colour and it clases with pink so green works well). You may want to paint their guns a different colour though Tyranid guns are just Tyranids bred to be guns so they could be the same colour as the rst of the model. A strong eye colour is important, paint the eye black and then apply a little moot green or yellow to make the eyes really stand out. A bit of bone and then white will tackle the teeth.
Hope this helps.
