There is the classic avatar that linden provides, the goal is to hide this avatar with alphas (these make body parts invisible) and replace those body parts with mesh (made by independant creators). If you go to your using folder in inventory and take everything off, that is the classic avatar.
Now, you need to build the avatar with pieces. Maitreya is the standard for the body because there are so many clothes for it, but it is mostly made for a skinny shape. For more curvacious shapes you need a slink hourglass or a belleza freya. There are bodies made that are more pluss side than those, but you will be extremly limited by the clothes. After you get your body, you need your head, there are different good creators for heads and you must take your time browsing them. You must be careful while buying a head since you could be stuck with a static head (not editable), you always need to make sure to only test the bento heads which you can edit with the sliders in the edit aparence on the avatar. Some examples of head creators are catwa, lelutka, laq, logo and genus. Genus and catwa might be the most popular ones
After you have your head you go for the skin. There are skins creators that include a soft chubby version of the skins for the bodies. Glam affair and Deetalez are my favorites and they have chubby options, but there are many more. You must make sure that when you land in a skin store, before buying the body skin (appliers meaning layers) they also have skins for the mesh head you decided to buy beforehand
BOM is a new sl system. Before we had mesh pieces made by independant creators, the people made skins for the classic avatars. With BOM now those skin can also be applied to the mesh bodies. I do not know if Belleza is already BOM ready, I only know of maitreya. When a mesh part is BOM, you don't need to hide the old avatar under it, the body does it and "becomes" the classic avatar accepting those old system layers.
After you have your head and body, you need to make a shape. Shapes are time consuming, you can spend hours in the avatar appearance (right click on the avatar) to make the new mesh parts and head look the way you like. If you do not want to spend time, you can look for shapes in the market, making sure they say the name of the head they used and it must be the same you bought. However, you should be careful with already made up shapes in the market, since the skins can change how a head looks and you would also need to buy the skin used by the person selling the shape (if you want an already made up shape, you must buy the shape before buying the skins)
After that, you have a bald and naked avatar you can buy things for (hair, clothes), clothes must always say that they are fitted for the body you bought. If some pieces still show through the clothes, you just need to use the body's hud (a window with buttons that the creator includes with the body) and it has options to make parts invisible. All clothes, regardless of how well they are made to fit a certain body, will still get disorted when the avatar moves, sit, etc; especially long sleeves or long pants.
Alpha: Layers you wear on the avatars to make parts invisible
Shape: Sliders in the avatar apearance menu to change avatar looks
Mesh: pieces that are made on outside programs, can be furniture, avatar clothes, avatar bodies, heads.
Layers: paint that is applied on the avatar, layers can be the skin, tattoes, underwear clothes, thigh fitted shirts, makeup and the hair base for the mesh hair not to look rootless.
If you are on a tight budget you have to be alert on offers. The other day, logo offered a free bento head for 1 linden. There are also some free mesh bodies around (of course you need to struggle with the clothes)