Im new

[(PSN)SuarezIndia1970]

I have so many mods dont know how to use them 

[Sir_Carl]

That's normal, I suggest you focus on the essentials. For weapons, damage and multishot (for guns) and then you can add some elemental mods (toxin, electricity, etc.). For Warframes, it really depends. For example, if you picked Excalibur, you want to have health because he is squishy, and then some strength and efficiency mods for his skills. 

My advice: don't try to learn everything at once, at lower levels, the game doesn't really punish you for making mistakes.

Edited July 29, 2019 by Sir_Carl

[CxLL]

Different enemies have different resistances and vulnerabilities. There is no “overall best way” to mod an item, nor can one item be the “very best” for every situation. This is why it is good to have a copy of all mods. Often piling up duplicates for various upgrade levels or being able to equip to more than just one weapon in the used loadout. Using a Codex scanner, available in your MARKET terminal for credits or a Synthesis scanner provided by cephalon Simaris on any Tenno-owned relay will allow you to complete CODEX entries which reveal the resistances, vulnerabilities, drop tables, locations and lore regarding each individual enemy the entry was completed for. So for instance, if you are going into a mission where there are a lot of enemies vulnerable to heat damage, you equip heat mods on your weapons.
Another sound advice would be paying attention to the weapon stats. Each weapon has a different set of numbers bound to its name, some higher, some lower than other weapons. Augmenting the highest numbers of  the weapon's stats will benefit the weapon. If you, for example, have a sword with a high Slash number but a low Impact number, you add slash mods. Sharpening an axe will do more than sharpening a hammer.

To upgrade an item, go to your arsenal, hover over the equipped item you wish to upgrade, select UPGRADE from the popup menu, and then drag mods from the list below into the slots above. If you wish to upgrade a mod, go to the MODS terminal, select the mod you wish to upgrade by clicking on it, followed by the click on the FUSION button in the upper left corner. Once in the Fusion menu, use the arrows under the mod to invest Endo into its levels.
If you wish to learn more about mods, modding, the Foundry and the Market segment of your Orbiter, you can read the Warframe Support website subsection Mods, Foundry and Market or consult the player-written info page on the Warframe fandom (wikia) environment (hotlink leads to the "Mods" page).

[(PSN)OS_Rflagg]

My only suggestion, don't waste time with Shield mods. 

Shields are static. They do NOT benefit from any resistance or damage reduction that you have acquired. Buff your health. Buff your armor. Buff your abilities if you tend to rely on them more than your weapons. Adaptation is a key mod. The more a damage type afflicts you, the higher the percentage the mod will protect against. 

Aside from that, build to your play style and how you want to play. 

If you want stealth, a "hush" mod only costs 3 points and turns rifles silent. 

[0_The_F00l]

That is not something that can be explained in one forum post, 

I suggest some "how to mod" videos as they will be more intuitive and easier to understand.